Nano-composite energetic powders prepared by arrested reactive milling

ABSTRACT

A method is disclosed for producing an energetic metastable nano-composite material as well as the energetic metastable nano-composite materials produced thereby. Under pre-selected milling conditions a mixture of powdered components are reactively milled. These components will spontaneously react at a known duration of the pre-selected milling conditions. The milling is stopped at a time at which the components have been compositionally homogenized to produce nanocomposite powder, but prior to said known duration, and thereby before the spontaneous reaction occurs. The milled powder is recovered as a highly reactive nanostructured composite for subsequent use by controllably initiating destabilization thereof.

RELATED APPLICATION

This application is a continuation in part of U.S. Ser. No. 12/381,514,filed Mar. 12, 2009 which is a division of U.S. Ser. No. 10/988,183,filed Nov. 12, 2004, now U.S. Pat. No. 7,524,355, which claims priorityfrom U.S. Provisional Patent Application Ser. No. 60/524,712, filed Nov.24, 2003.

GOVERNMENT LICENSE RIGHTS

The United States government may hold license and/or other rights inthis invention as a result of financial support provided by governmentalagencies in the development of aspects of the invention.

FIELD OF THE INVENTION

This invention relates generally to energetically reactive compositions,and more specifically relates to the preparation of metastablenano-composite materials by arrested reactive milling.

BACKGROUND OF THE INVENTION

A very high reaction enthalpy is an advantage of composite energeticformulations employing metallic fuels and solid oxidizers. However, therates of reaction of such formulations are limited by condensed phasetransport processes and are much lower than the reaction rates ofmono-molecular energetic formulations, e.g., TNT, HMX, RDX, etc. Currentresearch on composite energetics aims to reduce the length scaleslimiting mass transfer rates and to approach the reaction kineticsachievable for monomolecular formulations. Significant efforts have beenmade to produce metal fuel powders with nano-sized particles. (See e.g.,Dufaux, et al., Combustion and Flame 100 (1995) 350-358; Rosen, et al.,Chemical and Physical Processes in Combustion Combustion Institute,Raleigh, N.C., 1999, pp. 164-167) Faster burning rates have beendemonstrated with nano-sized metal powders for several applications,however difficulties in handling such powders and their incorporationinto existing formulations have also been reported (Brousseau, et al.,Propellants, Explosives, Pyrotechnics 27(5) (2002) 300-306). To benefitfully from the small length scale and large specific surface ofnano-sized metal particles, such particles should be intimately mixedwith similarly sized oxidizer particles. Progress in this direction hasbeen reported and nanopowders of metals and metal oxides have been mixedin organic solvents forming so-called Metastable IntermolecularComposites (MIC) (Bockmon, et al., CPIA Publication 712 (38th JANNAFCombustion Subcommittee Meeting) (2002) 613-624). Sol-gel processing hasalso been developed to produce nano-structured matrices of oxidizermaterials that can be filled with metallic nano-sized fuels (Tillotson,et al., Journal of Non-Crystalline Solids 285(1-3) (2001) 338-345).

In all of the current approaches, metal nanopowders have to besynthesized in a separate process and then mixed with the oxidizer.Handling of the highly reactive nano-sized fuel particles in closecontact with oxidizer is needed and some passivation is always required.In order to reduce ignition sensitivity of metal nano-powders,protective coatings are used (Maeng et al., Journal of Metastable andNanocrystalline Materials 15-16 (2003) 491-494), which typically reducethe overall enthalpy of the fuel. Nano-sized ingredient powders alsomake it difficult to achieve the desired high density of the finalenergetic formulations. In addition, the costs of synthesis,passivation, and handling of the fuel and oxidizer nanopowders arecurrently prohibitively high.

Commonly used energetic materials are based on monomolecular compounds,such as TNT, RDX, HMX, CL-20, etc. (Agrawal, Recent Trends in HighEnergy Materials Progress in Energy and Combustion Science 1998;24:1-30; Pagoria, et al., Thermochimica Acta 2002; 384:187-2041). Themaximum heat of combustion of such materials is generally limited by theenthalpy of formation of their reaction products, CO₂ and H₂O uponcomplete oxidation. The monomolecular energetic materials enable anexothermic reaction to occur very rapidly, with the rate controlledprimarily by the chemical kinetics processes for the moleculedecomposition (Tarver, Journal of Energetic Materials 2004; 22(2):93-107; Brill, et al., Kinetics and Mechanisms of Thermal Decompositionof Nitroaromatic Explosives Chemical Reviews (Washington, D.C., UnitedStates) 1993; 93(8):2667-92). On the other hand, the energy densities ofsuch materials are relatively low. Higher combustion energies and thushigher energy densities can be obtained from combusting metal fuels,such as Mg, Al, B, Ti, and others. The advantages of metal fuels areclear and become more significant when volumetric reaction enthalpiesare compared. The main drawback of using such fuels is associated withrelatively low rates of energy release. Micron-sized metal particlesignite after a fairly long delay as compared to the initiation ofmonomolecular energetic compounds. Such delays are usually controlled byrelatively slow heterogeneous reactions leading up to theself-sustaining combustion of the metal particles (Rozenband, et al.,Combustion and Flame 1992; 88 (1): 113-8; Trunov, et al. Combustion andFlame 2005; 140(4): 310-8; Trunov, et al. Combustion Theory andModelling 2006; 10(4):603-23; Kazakov, et al. Archivum Combustionis1987; 7(1-2):7-17). Furthermore, the rates of combustion of metalparticles are often not sufficiently high to fully utilize theirenergetic benefits in the applications involving explosives,propellants, and pyrotechnics. For micron-sized particles such rates arecommonly limited by the gas phase oxygen transport to the burningparticle surface.

Combinations of conventional, micron-sized metal powders with condensedoxidizers, such as relatively unstable metal oxides in thermitecompositions or ammonium perchlorate (AP) in solid propellants, do notresult in significant acceleration of the metal ignition and combustionrates compared to the metal ignition in gaseous oxidizers. Theheterogeneous processes controlling the metal ignition delays areusually associated with diffusion of oxidizer and/or fuel through theprotective layers of metal oxide. Such layers always form on the surfaceof the metal oxidizing at a low temperature (prior to its ignition) sothat the concentration of oxidizer outside the metal particle has only alimited effect on the rate of the critical diffusion processes. Inaddition, decomposition of the oxidizer typically occurs much soonerthan the metal particles ignite, so that the igniting and burning metalparticles are nearly always surrounded by a vapor-phase oxidizer. Thedelayed ignition often causes further problems, making metal combustionless efficient. For example, an issue of critical importance formetallized solid propellants is the agglomeration of unignited aluminumparticles (Price, Journal of Propulsion and Power 1995; 11(4):717-28;Babuk, et al., Journal of Propulsion and Power 2002; 18(4):814-23). Suchparticles, initially mixed with AP and binder, melt and agglomeratebefore they ignite. The resulting large size agglomerates may neverignite or they ignite after very long delays. Because of delayedignition, such agglomerates often cannot burn during the limited timethey fly through the propulsion chamber. Thus, a large portion of thealuminum additive remains unburned, reducing dramatically both theefficiency of the propulsion system and obtained specific impulse.

Similar issues also explain the very limited range of application ofconventional thermites. Initiation of a metal-metal oxide redox reactionis quite difficult for mixed micron-sized powders and requires extendedpre-heating of a relatively large or a well heat-insulated sample. Dueto the high thermal conductivity of metal-metal oxide mixture, small,poorly insulated samples lose heat very rapidly, and for such samplesthe initial heterogeneous reaction never becomes self-sustaining.

An idealized metal-oxidizer system similar to the monomolecularenergetic compound can be described: a metastable, homogeneousmetal-oxidizer solution in which the components are not chemicallybonded. Thus, the reaction rate would not be limited by heterogeneoustransport processes and can be dramatically accelerated. It was observedthat metastable metal-gas solutions form naturally inside combustingmetal particles (Dreizin, Progress in Energy and Combustion Science2000; 26(1):57-78; Dreizin, Combustion Explosion and Shock Waves 2003;39(6):92-96). Once such compounds form, they indeed react very rapidlyresulting in micro-explosions and disruptive particle combustion(Dreizin, et al., Combustion Science and Technology 1993; 90:79-99;Dreizin, et al., Combustion Science and Technology 1992; 87:45-58;Molodetsky, et al. Proceedings of the Combustion Institute (Twenty-SixthSymposium (Int'l) on Combustion). The Combustion Institute, Pittsburgh,p. 1919-1927, 1997; Molodetsky, et al., Combustion and Flame 1998;112:522-32). However, it is anticipated that a relatively strongchemical bonding occurs in such solutions which would limit their energydensity and thus respective practical applications.

It appears that a practically optimized metal-based energetic materialwould have the reactive components mixed on a scale as fine as possible,as long as significant chemical bonding between components is prevented.This naturally leads to the idea of using materials with high specificsurface area, or materials divided down to the nanoscale in order toreduce ignition delays and accelerate combustion of metals, as proposedby Danen, et al., U.S. Pat. No. 5,266,132; Nov. 30, 1993.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides nano-structuredcomposite energetic materials provided by using arrested reactivemilling of powdered mixture of metals and oxidizers for the metals, orby such milling of mixtures of starting solid components capable ofhighly exothermic reaction. The resulting materials are micron-sizedpowders that can be handled using conventional processing techniques.Each particle comprises a fully dense mixture of the reagents with athree-dimensional nanosized structure. The components may be, forinstance, a fuel and oxidizer, e.g., thermite compositions, or highlyreactive combinations of metals or metals and metalloids, e.g., B—Ti,B—Zr, Al—Ni or others. A fresh metal surface may be produced duringsynthesis but most of it is never exposed to oxidizing environment,unlike the fresh metal surface of the nanosized metal powders. Theinterface between the reactants may be formed at low temperatures andtherefore, passivating layers may not be produced on the fresh metalsurfaces to the same degree as for blends of nanometer sized powders orlayered nanocomposites produced at elevated temperatures by sputterdeposition (Blobaum, et al., Journal of Applied Physics 94(5) 20032915-2922).

Reactive Milling has been observed to trigger spontaneouscombustion-like reactions in a number of reactive systems. Such aspontaneous reaction is likely if the adiabatic reaction temperature ofthe components exceeds 1800 K (Takacs, Progress in Materials Science 47:355414 (2002); Munir, et al., Materials Science Reports, 3(7-8) (1989)277-365). Such spontaneous reactions show very high reaction ratesdesirable for many applications of energetic materials. In the presentinvention reactive milling is adapted to prepare highly reactivecomposites. Such materials are obtained just before the spontaneousreaction would have occurred during milling so that the reaction can becontrollably initiated later. In order to prepare such materials, thetime of spontaneous initiation must be known as accurately as possible.Current theoretical treatment of the milling process is not sufficientlyadvanced to predict initiation reliably. Therefore, in the process ofthe invention a parametric experimental investigation establishes theconditions leading to the spontaneous reaction for each composition ofinterest. This parametric investigation may not be necessary when thetheoretical treatment of the milling process is developed sufficientlyto predict initiation reliably.

The nano-structured composite energetic materials of the presentinvention are preferably powders having composite particles and featureinterfaces between the composite components that are different from theinterfaces of materials previously available. The nano-structuredcomposite energetic materials of the present invention display uponheating different heat flow traces recorded by differential scanningcalorimetry (DSC) than materials with identical chemical compositionsprepared by other means and available previously. In some embodiments,the nano-structured composite energetic materials of the presentinvention undergo an exothermic reaction more quickly or at a lowertemperature than materials previously available. In some embodiments,the nano-structured composite energetic materials of the presentinvention undergo an exothermic reaction in a time that is about 10%,20%, 25%, 30%, 40% or 50% or more shorter than the time required formaterials previously available to undergo a similar or the same reactionat a given heating rate. In some embodiments, the nano-structuredcomposite energetic materials of the present invention begin to react inan exothermic reaction at a temperature that is at least about 10%, 15%,20%, 25%, 30%, 40% or 50% or more lower than the temperature at whichmaterials previously available begin to react in the same or a similarreaction at a given heating rate. In other embodiments, thenano-structured composite energetic materials of the present inventionbegin to react in an exothermic reaction at a temperature that is about25, 50, 60, 70, 80, 90, 100, 125, 150, 175, 200, 225, 250, 275, 300 ormore degrees Kelvin lower than the temperature at which materialspreviously available begin to react in the same or a similar reaction ata given heating rate. In still other embodiments, the nano-structuredcomposite energetic materials of the present invention react in anexothermic reaction with a different number of peaks recorded by DSC andsubstantially different peak shape of the reaction curve recorded by DSCas compared to the materials previously available and reacting in thesame or similar reaction at a given heating rate. In yet otherembodiments, the nano-structured composite energetic materials of thepresent invention react in an exothermic reaction at a lower temperatureand the reaction occurs more actively than the materials previouslyavailable prepared by other means. That is, in these embodiments, theexothermic features are broad and individual peaks may be difficult orimpossible to separate. In such embodiments, the nano-structuredcomposite energetic materials of the present invention may react in anexothermic reaction with a broad exothermic feature starting at about400°, 450° or 500° K and increasing up to about 800°, 850°, 900° or 950°K.

The nano-structured composite energetic materials of the presentinvention are preferably in powder form with composite powder particlesfeaturing interfaces between the composite components that are differentfrom the interfaces of materials previously available. As such, particlesizes and inclusions are different from materials produced by othermeans. In preferred embodiments, most or all of the particles arebetween 1-100 microns in diameter. In other embodiments, at least abouthalf of the particles are smaller than about 50 microns in diameter, orat least about half of the particles are smaller than about 40 microns,30 microns or 20 microns in diameter. In some embodiments, most or allof the inclusions in the material are about 10-1,000 nanometers indiameter. In other embodiments, most or all of the inclusions in thematerial are about 30-300 nanometers in size, or 40-250 nanometers indiameter or 50-200 nanometers in diameter. The nano-structured compositeenergetic materials of the present invention are denser than similarpowdered materials produced by other means. They may be, for instance,at least about 10%, 15%, 20%, 25%, 30%, 40%, 50%, 60%, 70%, 80% or moredenser than similar powdered materials of the same elements produced byother means. In some embodiments, they may even be two or three or moretimes denser than similar powdered materials of the same elementsproduced by other means.

In preferred embodiments, the nano-structured composite energeticmaterials of the present invention are powders, and they undergo morerapid ignition than similar materials of the same elements produced byother means at a given heating rate. In some embodiments, thenano-structured composite energetic materials of the present inventionsubstantially complete an ignition reaction in at least about 5%, 10%,15%, 20%, 25%, 30%, 40%, or even 50% or more less time than the timerequired for similar materials of the same elements produced by othermeans at a given heating rate to substantially complete an ignitionreaction. In some instances, the nano-structured composite energeticmaterials of the present invention may substantially complete anignition reaction in about 500, 600, 700, 800, 1000, 1200, 1400, 1500,1600, 1800 or 2000 μsec.

In preferred embodiments, the nano-structured composite energeticmaterials of the present invention are powders that can be mixed withbinders, such as polymers, organic solutions, or other materials, andthe composite flow behavior of the resulting mixtures is substantiallydifferent than the composite flow behavior of similar mixtures usingidentical binders and powder additives having chemical reactivitiessimilar to those of the nano-structured energetic materials. In someembodiments, the composite flow viscosity of the powder-binder mixturesprepared using nano-structured composite energetic materials of thepresent invention is substantially lower than the composite flowviscosity of mixtures prepared using identical binders and powderadditives having chemical reactivities similar to those of thenano-structured energetic materials. In some embodiments, the compositeflow viscosity for the powder-binder mixtures prepared usingnano-structured composite energetic materials of the present inventionis at least about 5%, 10%, 15%, 20%, 25%, 30%, 40%, or even 50% or moreless than the composite flow viscosity of mixtures prepared usingidentical binders and powder additives having chemical reactivitiessimilar to those of the nano-structured energetic materials.

In a second aspect, the present invention provides a method forproducing an energetic metastable nano-composite material. Underpre-selected milling conditions a mixture of powdered components isreactively milled. These components will spontaneously react after aknown time duration specific for the pre-selected milling conditions.The milling is stopped at a time at which the components have beencompositionally homogenized to produce nanocomposite particles, butprior to said known duration, and thereby before the spontaneousreaction occurs. The milled powder is recovered as a highly reactivenano-structured composite for subsequent use by controllably initiatingdestabilization thereof. During recovery and handling, only the externalsurface of the particles is exposed to oxygenated environment andpassivated by oxidation. Most of the freshly produced reactive metalinterfaces remain within the nanocomposite material and retain theirhigh reactivity.

In a preferred procedure for practicing the invention the milling iseffected in a ball mill, where the pre-selected conditions include thenumber of balls; the ball diameter and ball material; the ratio betweenthe mass of balls and the mass of the said components; the material,size, and shape of the milling container; the temperature of the millingcontainer, the presence or absence of milling aids such as surfactants;and the mill operating parameters used during the milling operation,which for example in a vibratory ball mill would include oscillatingrate and amplitude. On the other hand, in a stirred mill such as the“Attritor,” rotational speed of the stirrer is such an operatingparameter. Similarly, in a planetary mill rotational speed of themilling vial is such an operating parameter.

In some embodiments, the present invention features a method forsystematically producing an energetic metastable nano-composite materialby (a) reactively milling a mixture of powdered components thatspontaneously react at a known duration of said milling; (b) stoppingsaid milling at a time at which said components are compositionallyhomogenized on a nanoscale to produce a nanocomposite powder, but priorto said known duration, and thereby before said spontaneous reactionoccurs; and (c) recovering as a product the milled powder as ananostructured composite for subsequent use by controllably initiatingdestabilization thereof. The milling may be effected in a ball mill, andthe pre-selected conditions may include the number of balls, the balldiameter and ball material, the ratio between the mass of balls and themass of the components, the material, size, and shape of the millingcontainer, the temperature of the milling container, the presence orabsence of milling the compositions, and the mill operating parametersused during the milling. In some embodiments, the reactive milling maybe affected in a vibratory mill in which the containers for the productsbeing milled is agitated at high frequency in a complex cycle based onmotion in three orthogonal directions. In some embodiments, reactivemilling is effected in a stirred mill. In some embodiments, reactivemilling is effected in a planetary mill. In some embodiments, reactivemilling is effected in a planetary mill with the milling containerscooled by an air-conditioned air flow. In some embodiments, reactivemilling is effected in an attritor mill with the milling containercooled by liquid nitrogen. Likewise, the known duration may beexperimentally determined for the components subjected to milling. Insome embodiments, the components comprise a metal and an oxidizer forthe metal. The components may be at least a pair of reactive metals. Thecomponents may contain at least a pair of thermite reactants. In someembodiments, the recovered product may contain particles in the 1-50 μmrange. In some instances, the adiabatic reaction temperature of thecomponents exceeds 1800 K. The present invention also provides theproduct produced by the methods described.

In some embodiments, the methods feature a method for systematicallyproducing an energetic metastable nano-composite material by (a)selecting starting components as two or more powdered materials capableof a highly exothermic reaction; (b) reactively milling the startingcomponents to achieve homogeneity; (c) stopping said milling at a timeat which the components are compositionally homogenized on the nanoscaleto produce a nanocomposite powder, but prior to initiation of anexothermic reaction; and (d) recovering as a product the milled powderas a metastable nano-composite for subsequent use by controllablyinitiating destabilization thereof. In some instances, the milling isperformed under pre-selected milling conditions providing transfer ofenergy from milling tools to the powdered components required to producehomogeneity. Likewise, in some instances the milling is performed underpre-selected milling conditions according to t=const/C_(R) where t ismilling time, C_(R) is a charge ratio defined as a ratio of mass ofmilling tools to mass of the powdered components and const is a constantdepending upon type of milling equipment. The present invention alsoprovides the product produced by these methods described as well.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further illustrated in the Figures appended hereto, inwhich:

FIG. 1 schematically illustrates experimental configurations used forpreliminary ignition and combustion tests of the synthesized energeticnano-composite powders: (a) Heated filament ignition; (b) Constantvolume explosion; (c) Linear burning rate.

FIG. 2 are graphs of experimental data on reactive milling of Al—MoO₃and Al—Fe₂O₃ compositions presented as the product of mixture ratio andmilling time plotted versus ball diameter.

FIG. 3 shows SEM images of the prepared reactive nano-composite powdersand respective particle cross-sections.

FIG. 4 depicts graphs for particle size distributions of partially andfully milled nanocomposite powders. Unmilled blends of startingmaterials are shown for comparison.

FIG. 5 is an arrhenius plot of ignition temperatures of the fully milledAl—MoO₃, Al—Fe₂O₃, and B—Ti nano-composites presented in Table 2 herein.Similarly measured data for ignition of Al and Mg powders in air arealso shown.

FIG. 6 shows pressure traces recorded in the constant volume explosionexperiments in air with the loaded fully and partially milled Al—MoO₃and Al—Fe₂O₃ nano-composite powders. The summary of the measuredpressures and rates of pressure rise are given in Table 3 herein.

FIG. 7 is a backscattered electron image showing combustion products ofthe fully milled Al—MoO₃ nano-composite recovered from the constantvolume vessel; and

FIG. 8 is a graph depicting results of the linear burn tests conductedwith fully and partially milled Al—Fe₂O₃ nano-composite powders.

FIG. 9 is a typical TEM structure of the nano aluminium particleproduced in different atmospheres: (I) helium, (II) argon, and (III)nitrogen: (a) 0.025 MPa, (b) 0.05 MPa, and (c) 0.1 MPa.

FIG. 10 is a scanning electron microscopy images of different thermitecompositions prepared by mechanical mixing of starting nano-sizedcomponents in a liquid solvent: a) Al—MoO₃ b) Al—Bi₂O₃; c) Al—WO₃; d)Al—CuO (100).

FIG. 11 provides an SEM image and elemental distribution images ofas-deposited foil. The substrate side of the foil is the left side ofthe Al layer. The right side of the Al layer is the top side. (a) Brightfield SEM image. (b) Cu map (L_(2,3) edge, 931 eV) (c) O map (K edge,535 eV). (d) Al map (K edge, 1560 eV) (117).

FIG. 12 is a cross-section of a nanocomposite reactive particle withbulk composition 8Al+3CuO (metal-rich thermite composition). The imageis taken with backscattered electrons and shows phase contrast betweenCuO (light inclusions) and Al (dark matrix).

FIG. 13 provides a comparison of the particle size distributionsdetermined by SEM, SAXS and SANS analyses of a commercially availablenano-aluminum powder produced by Technanogy (sample ID: TN-Al-40) (147).

FIG. 14 is a high-resolution electron micrograph of an aluminumnanoparticle showing layered structure in oxide/hydroxide surface layerand the aluminum/oxide boundary. The oxide film is generally amorphouswith inclusions of discontinuous crystalline layers (image in thecenter). Exfoliation of single molecular thickness laminar sheets froman oxide coating is illustrated in the image on the right (150).

FIG. 15 provides experimental and computed DSC curves showing melting ofaluminum nanopowders (146). Each plot is labeled with nominal particlesize.

FIG. 16 provides DSC and TGA curves measured for three aluminumnanopowders heated in argon and oxygen, respectively. Both sets ofcurves were acquired at a heating rate of 5° C./min (146).

FIG. 17 demonstrates change in mass of the aluminum powder oxidizing ina thermal analyzer. Different stages of oxidation are indicated and therespective changes in the growing alumina scale are shown schematically(7).

FIG. 18 demonstrates TGA and DSC traces for reacting aluminum-copperoxide nanocomposite materials prepared as multilayer nanofoils (116,117) and as fully dense nanocomposite powders (177), respectively. Bothtraces recorded in flowing argon at 40 K/min.

FIG. 19 demonstrates DSC traces for reacting aluminum-molybdenum oxidenanocomposite materials prepared as mixed nanopowders (170) and as fullydense nanocomposite powders (178), respectively. Both traces recorded inflowing argon at 5 K/min.

FIG. 20 demonstrates ignition delays measured for laser ignition ofAl—MoO₃ thermite powders prepared with aluminum particles of differentsizes (158).

FIG. 21 provides results of experiments on ignition of different powdersin heterogeneous shock tube experiments (196). Emission intensity at 486nm is shown versus time for four energetic materials in 30% O₂/70% N₂.The ambient temperature is 2250 K and the ambient pressure is 3.0 atm.The inset shows a magnified temporal region soon after endwallreflection.

FIG. 22 provides isoconversion analysis of DSC and ignition data fornanocomposite powders with bulk composition 2Al+MoO₃ prepared by ARM(178).

DETAILED DESCRIPTION OF THE INVENTION Manufacturing of ReactiveNanopowders Nanosized Aluminum

Nanosized aluminum powder or nanoaluminum (n-Al) is the most commoncomponent of metal-based reactive nanomaterials, while othernanopowders, e.g., boron, magnesium, or zirconium have also beenconsidered (Kuo, et al., Materials Research Society SymposiumProceedings, 800 (Synthesis, Characterization and Properties ofEnergetic/Reactive Nanomaterials) Materials Research Society, p. 3-14,2003). The popularity of n-Al is understandable because it is oftenconsidered as a potential replacement for the conventional aluminumpowders and flakes widely used in explosives, propellants, andpyrotechnics. In fact, the rapid acceleration of research in the area ofreactive nanocomposite materials can be readily traced to thedevelopment in the n-Al manufacturing and to the time it becameavailable for experiments more than a decade ago. The methods ofmanufacturing n-Al can be broadly classified into those involving vaporphase condensation and liquid phase chemistry.

One of the first methods described in the literature was production ofn-Al by exploding electrically heated wires, pioneered by Russianscientists (Zelinskii, et al., Fizika i Khimiya Obrabotki Materialov1984; (1):57-9 (in Russian); Yavorovsky, Patent of Russian FederationNo. 2048277, 1995; Yavorovsky, et al., Patent of Russian Federation No.2048278, 1995; Ivanov, et al., Pyrotechnics 2003; 28(6):319-33). Thismethod continues to be developed around the world (Jiang, et al., IEEETransactions on Plasma Science 1998; 26(5): 1498-501; Kwon, et al.,Scripta Materialia 2001; 44:2247-51; Sarathi, et al., MaterialsCharacterization 2007; 58:148-55) and a large portion of publicationsdescribing production of n-Al are related to this method. The producedpowders were branded as Elex or Alex (Ivanov, et al., Kuo, editor.Challenges in Propellants and Combustion 100 Years after Nobel, BegellHouse, New York; 1997, p. 636-45; Tepper, Powder Metallurgy 2000;43(4):320-2) and are available commercially. Aluminum nanoparticles arealso produced from condensing aluminum vapor generated when a thinaluminum wire is vaporized by a strong electric current passing throughit. Nanoparticles of many other metals and alloys were also obtained(Tepper, Metal Powder Report 1998; 53(6):31-33; Kwon, et al.,Proceedings—9th Russian-Korean International Symposium on Science andTechnology, KORUS-2005 1, art. no. 1507688, 2005, p. 211-3; Wang, etal., Materials Science and Engineering 2001; A307:190-4). Characteristictransmission electron microscope (TEM) images of aluminum nanopowdersobtained by exploding electrically heated wires are shown in FIG. 9. Itwas proposed that the required current density should exceed 10¹⁰ A/m²(Kwon, et al., Scripta Materialia 2001; 44:2247-51). It was alsoproposed that the average size of the produced particles is inverselyproportional to the cube of the energy released into the wire (Rossi, etal., Journal of Microelectromechanical Systems 2007 16(4): 919-31; Kwon,et al., Scripta Materialia 2001; 44:2247-51). In various investigations,the characteristic wire diameter varied from tens to hundreds ofmicrons. Specific electric circuits designed to produce such pulses weredescribed in many publications (Romanova, et al., Czechoslovak Journalof Physics 2006; 56(Suppl. B):B349-B356; Sedoi, et al., IEEETransactions on Plasma Science 1999; 27(4):845-50; Chemezova, et al.,IEEE 18th Int. Symp. on Discharges and Electrical Insulation inVacuum-Eindhoven-I998, p. 48-5132-34). A high-voltage source (5-30 kV)is commonly used to produce a current pulse on the order of severalthousands of amperes. The pulse duration varies from nanoseconds(Romanova, et al., Czechoslovak Journal of Physics 2006; 56(Suppl.B):B349-B356), achieved using customized electronic components, toseveral microseconds (Jiang, et al., IEEE Transactions on Plasma Science1998; 26(5):1498-501) produced using common L-C circuits. The underlyingphysics of the wire explosion remains the subject of currentinvestigations but is outside the scope of this paper. For preparationof nanopowders, the wire explosion is commonly set up in an inert gasenvironment. The particles are collected on the walls of the explosionvessel. The sizes of the obtained particles typically vary over a fairlybroad range—from 10-20 nm to microns. The effects of pressure, gasenvironment, electric pulse characteristics, and other experimentalparameters were studied (Zelinskii, et al., Fizika i Khimiya ObrabotkiMaterialov 1984; (1):57-9 (in Russian); Jiang, et al., IEEE Transactionson Plasma Science 1998; 26(5):1498-501; Kwon, et al., Scripta Materialia2001; 44:2247-51; Sarathi, et al., Materials Characterization 2007;58:148-55; Ivanov, et al., Kuo, editor. Challenges in Propellants andCombustion 100 Years after Nobel, Begell House, New York; 1997, p.636-45). Jiang, et al., IEEE Transactions on Plasma Science 1998;26(5):1498-501 established that a higher pressure results in theformation of coarser particles. On the other hand, it was reported thatan increased pressure of the inert gas results in an increased yield ofaluminum nanoparticles (Sarathi, et al., Materials Characterization2007; 58:148-55). Aluminum nitride was formed in the experimentsperformed in nitrogen (Sarathi, et al., Materials Characterization 2007;58:148-55; Kwon, et al., Applied Surface Science 2003; 211; 57-67). Themodifications of this technique include automation of the wire feed intothe discharge region, setting up a flowing gas system, and altering thegas environment to passivate the aluminum nanoparticles (Tepper, PowderMetallurgy 2000; 43(4):320-2; Tepper, Metal Powder Report 1998;53(6):31-33). In particular, treatment of the nanopowders inenvironments with low partial pressure of oxygen (e.g., 0.01% of thetotal pressure) results in the formation of protective oxide coatingswhich effectively stop further oxidation of nanoparticles during theirhandling and storage. Passivation is commonly accomplished as a separateprocessing step, in which the chamber filled by inert gas for powderproduction is evacuated and refilled with an oxidizing gas mixture.Other approaches have also been considered in which the powders werepassivated by fluoropolymers, stearic and oleic acids, and aluminumdiboride (Kwon, et al., Applied Surface Science 2003; 211; 57-67;Gromov, et al., Powder Technology 2006; 164: 111-5). The advantages ofpreparing n-Al from electro-exploded wires include the method's relativesimplicity and the efficient use of electric energy. The maindisadvantages are the relatively low production rate and difficulties inobtaining the product powders with a useful narrow size distribution.

Other techniques for the production of n-Al, including evaporation ofbulk aluminum samples or aerosolized micron-sized powders followed bycontrolled vapor condensation, have been discussed in the literature(Granqvist, et al., Journal of Appl. Phys. 1976; 47(5):2200-19;Puszynski, Proceedings of the 29^(th) International PyrotechnicsSeminar. Publisher: Defense Science & Technology Organization,Pyrotechnics Group; 2002, p. 191-202; Schefflan, et al., Journal ofEnergetic Materials 2006; 24(2): 141-56; Pivkina, et al., Journal ofThermal Analysis and Calorimetry 2006; 86(3): 733-8; Munz, et al., PureAppl. Chem. 1999; 71(10):1889-97; Phillips, et al., U.S. Pat. No.6,689,192 B 1 20040210, 2004; Champion Y. Annales de Chimie (Cachan,France) 2006; 31(3); 281-94; Miziolek, The AMPTIAC Newsletter 2002;6(1):43-8) and some of them were commercialized. Commonly, condensationin a low pressure (less than 10 torr or 1.3 kPa) inert gas results inthe formation of nanoparticles, while higher pressures result inincreased particle sizes. Condensation of metal vapors in lighter inertgases (e.g., He vs Ar or Xe) results in respectively finer particlesizes (Granqvist, et al., Journal of Appl. Phys. 1976; 47(5):2200-19).Most commonly, a crucible containing bulk aluminum is heated in aflowing inert gas environment. Radiative heaters, induction heaters,lasers, electric arcs, or special high temperature furnaces have beenused to vaporize the bulk aluminum sample. Of particular interest aredevices using arc or induction plasma to evaporate precursor materialand to further use the plasma flow to transfer the superheated vapor tothe quenching zone (Boulos, et al., US Patent Application 20070029291,2007; Choi, et al., Repub. Korean Kongkae Taeho Kongbo 2008). The Alvapor is carried out into a passivation section where the inert gas ismixed with a small amount of oxygen. Finally, the passivated powder iscollected thermophoretically. Miziolek, The AMPTIAC Newsletter 2002;6(1):43-8) show aluminum wire fed into a vacuum chamber and evaporatedfrom a heated ceramic boat. The condensation occurred in a helium orargon gas stream at pressures in the range of 2-16 torr. Many processingdetails remain proprietary and were never published. Other examples ofdevices producing n-Al are described in refs. (Tepper, Powder Metallurgy2000; 43(4):320-2) and (Granqvist, et al., Journal of Appl. Phys. 1976;47(5):2200-19).

Particle growth from condensing vapor has been the subject of multipletheoretical studies, some of which specifically focus on Al, e.g.,(Panda, et al., NanoStructured Materials 1995; 5(7/8):755-67). It wasshown that nanoparticles form in a relatively narrow temperature range.Finer particles are produced at higher cooling rates. In addition, lowevaporation temperature, low pressure or low metal vapor concentrationare desired. Particle nucleation and growth were consideredtheoretically (Puszynski, et al., Proceedings of the 4^(th) WorldCongress on Powder Technology, Sydney, Australia, Paper 164, 2002) andthe effect of different gas environments was explored. It was suggestedbased on both experiments and calculations, that carrying out thesynthesis in helium results in much finer size particles compared tousing argon. It was further confirmed that higher pressures result incoarser particles. In another study, it was found that coalescence ofcolliding nanoparticles results in a substantial reduction of thesurface energy and resulting temperature increase of the productparticle (Lehtinen, et al., Aerosol Science 2002; 33:357-68). Thistemperature increase affects the final dimensions of the growingnanoparticles (Mukherjee, et al., Journal of Chemical Physics 2003;119:3391-404). There are now numerous theoretical papers on nanoparticlecondensation from vapors, covering a variety of metals and operatingconditions.

An example of using modeling for the design of nanopowder producingequipment is presented by Schefflan, et al., Journal of EnergeticMaterials 2006; 24(2): 141-56. Particle growth by nucleation andcoagulation is considered theoretically. The problem is solvednumerically. Numerical solutions offer the advantage of exploring theeffects of specific, system-dependent temperature profiles, gasconcentrations, etc. For example, the effect of different temperatureprofiles on the product particle size distribution is reported in(Schefflan, et al., Journal of Energetic Materials 2006; 24(2): 141-56).

Modifications of the generic approach based on physical vaporcondensation include alloying or doping aluminum with various additives.For example, preparation of n-Al with added barium is described byPivkina, et al., Journal of Thermal Analysis and Calorimetry 2006;86(3): 733-8. Such readily oxidizing additives reportedly increase thereactivity of the produced aluminum and enable it to react at lowertemperatures as compared to the pure powders. Similar to the n-Alproduced by wire explosion, post-processing steps are used to passivatethe surface of the nanoparticles. Both treatment in low-pressure gaseousoxygen environment and processing powders in various liquid solutions(such as oleic acid and phenyltrimethoxysilane (Puszynski, et al.,Materials Research Society Symposium Proceedings 2006; 896:147-58)) havebeen reported.

Variations of the bulk aluminum heating technique were reported in byPuszynski, et al., Materials Research Society Symposium Proceedings2006; 896:147-58 and Park, et al., J. Physical Chemistry B. 2005;109(15):7290-9) in which aluminum was heated in an arc discharge orablated by a Nd-YAG laser. The nanoparticles produced were introducedinto cold argon environment for laboratory measurements which did notrequire substantial amounts of material. In laboratory scalemeasurements the particle size distributions were well-controlled.However, currently, scaling up the n-Al production using such heatingtechniques remains impractical.

Coating of magnetic metallic particles with carbon was reported for thearcs operated in a flow of methane (Dong, et al., J. Appl. Phys. 1999;86(12):6701-6; Zhang, et al., Journal of Physics: Condensed Matter 2001;13(9):1921-9). In a modification of the latter approach, aluminum wasused as a consumable anode of the microarc discharge operated in naturalgas to produce aluminum nanoparticles coated with a thin protectivelayer of carbon (Ermoline, et al., Nanotechnology 2002; 13:638-43).Carbon coated aluminum nanopowders were also obtained when aluminumnanoparticles generated by arc- or laser ablation of an aluminum targetwere quenched in an argon-ethylene flow (Park, et al., Journal ofNanoparticle Research 2006; 8: 455-64). Single-particle massspectrometry measurements reported in (Park, et al., Journal ofNanoparticle Research 2006; 8: 455-64) showed that carbon coatingsremain stable at the temperatures exceeding 900° C. and thus areattractive for protection of n-Al powders. Unfortunately, the reportedproduction rates for carbon-coated n-Al powders were very low and notyet suitable for the manufacture of such powders.

Flame synthesis is among the most exotic vapor condensation techniquesreported for the preparation of metallic nanopowders (Rosner, Eng. Chem.Res. 2005; 44:6045-55; Calcote, et al., Twenty-Fourth Symposium(International) on Combustion, The Combustion Institute, Pittsburgh,1992, p. 1869-76; Wooldridge, Progress in Energy and Combustion Science1998; 24(1):63-87; Steffens, et al., Chemistry of Materials 1996;8(8):1871-80; Axelbaum, Powder Metallurgy 2000; 43(4):323-5). The flameenvironment is adjusted so that the produced metal particles do notoxidize immediately. In refs. (Calcote, et al., Twenty-Fourth Symposium(International) on Combustion, The Combustion Institute, Pittsburgh,1992, p. 1869-76; Steffens, et al., Chemistry of Materials 1996;8(8):1871-80; Axelbaum, Powder Metallurgy 2000; 43(4):323-5), the metalparticles are immediately coated by salts also produced in the flame.While this coating is protective and can be readily removed bydissolution, it is not thin and, in fact, comprises a substantial volumeportion of the bulk product. Therefore, using such nanoparticlesembedded in a matrix of salt is impractical for most energetic materialapplications.

Wet chemistry techniques are attractive for the commercial synthesis ofaluminum nanopowder because of the inherent safety of handling thereactive powder under liquid and the ability to readily functionalizethe particle surface. However, we are unaware of any aluminumnanopowders produced by wet chemistry techniques on the commercial or ona practically interesting scale. Higa et al., U.S. Pat. No. 6,179,899nanosized aluminum powders were prepared by decomposing alane-adducts inorganic solvents under an inert atmosphere. Effective adduct specieswere reported to include trialkyl amines, tetramethylethylene-diamine,dioxane, and other aromatic amines and ethers. Reportedly, highlyuniform particles were obtained with particle size selectable in therange of about 65-500 nm by adjusting the catalyst concentration and byvarying the concentration of the adduct species. As typical for allreported wet chemistry techniques, the methodology is based on carefuland slow mixing of measured amounts of the starting solutions followedby continuous stirring and drying the product. Such a methodology is notwell suited for scaled up production and substantial modifications arenecessary to obtain practical quantities of the desired reactivenanopowders.

Despite these limitations of the wet-chemistry approach, surfacepassivation remains an important safety and handling issue and currentresearch is focused on preparing high quality and well-passivatedpowders (Foley, et al., Chemistry of Materials 2005; 17(16): 4086-91;Jouet, et al., Chemistry of Materials 2005; 17(11):2987-96; Jouet, etal., Materials Science and Technology 2006; 22(4):422-429). For example,surface layers of transition metals were formed on Al nanoparticles toprevent them from oxidation in surrounding air (Foley, et al., Chemistryof Materials 2005; 17(16): 4086-91; Gao, Chinese Patent CN 101041180 A20070926, 2007). Foley, et al., Chemistry of Materials 2005; 17(16):4086-91) report aluminum nanopowder was synthesized by thermaldecomposition of an alane solution in the presence of a titaniumcatalyst under an inert atmosphere. Gao, Chinese Patent CN 101041180 A20070926 report the nanopowder was formed upon mixing and drying ofaluminum dissolved in NaOH co-mixed with a nickel salt solution. In bothcases, aluminum nanoparticles served as a reducing agent for thetransition metal complexes, so that reduced metal films were produced onthe aluminum surface. In a different passivation approach, aluminumnanopowders coated with non-metallic self-assembled monolayers (SAMs)were prepared (Jouet, et al., Chemistry of Materials 2005;17(11):2987-96; Jouet, et al., Materials Science and Technology 2006;22(4):422-429). Nanoscale Al particles were prepared in a solution bycatalytic decomposition of the Al-methylpyrrolidinealane adduct bytitanium(IV) isopropoxide; followed by in situ coating using organicSAMs (e.g., diethyl ether solutions of perfluorotetradecanoic acid,perfluorononanoic acid or perfluoroundecanoic acid). The protectivelayer is clearly fairly thick resulting in the overall reduction of theenergy density of such materials.

A sonochemical approach was adapted to synthesize aluminum nanopowders(Harruff, et al., South-Eastern Regional ACS meeting, Greenville, S.C.October, 2007). In this technique, a solution is prepared in whichacoustic cavitation is induced. The cavitation results in the formationof small regions heated to about 5000 K, which cool very rapidly, at therates up to 10¹⁰ K/s. Volatile compounds trapped in these regionsdecompose and aggregate to form particles with characteristic dimensionsof about 10-60 nm. The early efforts aimed at production of reactiven-Al appear to be promising because of the potential capabilities tocontrol the product properties and scale the production up.

New approaches to passivating the surface of n-Al continue to bedeveloped and substantial progress is expected in this area in the nearfuture. For example, encapsulation of aluminum nanopowders inpolystyrene was recently described by Zhang, et al., HannengCailiao/Chinese Journal of Energetic Materials 2007; 15 (5):482-4 andwas shown to be effective in preventing aluminum oxidation. An approachsimilar to that proposed by Wilson, et al., Journal of Vacuum Scienceand Technology A: Vacuum, Surfaces and Films 2008; 26(3): 430-7resulting in the formation of atomic layer deposition of refractorymetal on surface of metallic nanoparticles may also be of interest inthe future.

Additional Nanopowders

Highly exothermic intermetallic, metal-metalloid, and thermite reactionsare commonly exploited in metal-based reactive nanomaterials. Additionalnanoscale components include metals other than aluminum (Mg, Zr, Ti, Ni,etc.), boron (a metalloid), and metal oxides. Recently, nano-sizedsilicon powders and nanoporous silicon wafers were considered forenergetic applications (Son S. F. Personal communication, 2008). Poroussilicon wafers are typically prepared by electrochemical etching of Siwafers in different electrolytes (Sun, et al., Advanced Materials 2007;19(7): 921-4). Because of multiple applications in electronics andelectro-optical devices, synthesis of Si nanoparticles was addressed bymany researchers in the past. Some of the related techniques aredescribed by El-Shall, et al., 2003 Journal of Physical Chemistry B 107(13), pp. 2882-2886; Makimura, et al., 2002 Japanese Journal of AppliedPhysics, Part 2: Letters 41 (2 A), pp. L144-L146 and Dang, et al., 2007ECS Transactions 2 (7), pp. 255-265). Use of microwave plasma reactorsfor production of silicon nanoparticles should be mentioned inparticular because of substantial practical benefits and potential forthe scaled up production (Giesen, et al., Journal of NanoparticleResearch 2005, 7(1): 29-41).

Production of most metal nanopowders is accomplished using one or moreof the same techniques described above for aluminum. Such powerfulenergy sources as lasers or arcs make it possible to vaporizepractically any metal and thus enable vapor-condensation basedtechniques. Reduction of various metallic complexes in solutionrepresents another common approach for generating metal nanopowders. Inboth cases, the cost of the nanopowders is relatively high and theproduction rates are limited. Commercially available boron powders havenot been marketed as nanopowders, but they typically have primaryparticles with sizes of the order of 100 nm. The fine primary particlesare strongly agglomerated while the specific surface of the material issimilar to that of a spherical nanosized powder. The surface features ofindividual particles are very fine, but the particle shapes areirregular. In order to achieve effective mixing of such a powder withother components, advanced mixing techniques are required.

Metal oxides are often available commercially in various size ranges andthus need not be always produced specifically for preparation ofreactive nanocomposite materials. At the same time, development ofoptimized and customized technologies for preparation of nanosized oxidepowders remains an active research area. For example, detailedprocedures for preparing nano-sized MoO₃ are described by Khan, et al.,WO 02/38239 A1. The process is based on sublimation of a precursormaterial followed by the controlled condensation of MoO₃. Thecondensation occurs in a stream of oxygen-rich gas to enable completeoxidation of the condensing molecules. A variant of this approach isdescribed by Mitra, et al., Thin Solid Films 2008; 516:798-802 where athin, electrically heated metal filament is used as the source of metalvapor.

Many nano-sized oxide powders were readily produced by chemical vapordeposition-based techniques. In particular, various flame synthesisapproaches (Rosner, Eng. Chem. Res. 2005; 44:6045-55; Pratsinis,Progress in Energy and Combustion Science 1998; 24(3):197-219; Skandan,et al., Nanostructured Materials 1999; 11 (2):149-158) are particularlywell suited for producing nanoparticles of various oxides withcontrolled properties. Usually, metalorganic precursors are injectedinto a fuel/air mixture. The precursor materials decompose and theproduced metals oxidize in a well-controlled flame yielding a uniformnano-sized oxide powder product. More recently, the formation ofnarrow-sized and specifically shaped nano-oxide particles or nano-fiberswas achieved by flame synthesis performed over specifically preparedsubstrates introduced in well-controlled flat flames (Xu, et al.,Applied Physics Letters 2006; 88(24) art. no. 243115). Alternately,external electric fields can be used to control the formation of oxidenanoparticles and their agglomeration (Zhao, et al., Journal ofNanoparticle Research, 2007; 1-17).

Microwave plasma reactors are also widely used for preparation ofnano-sized oxide and composite particles (Vollath, et al., Journal ofthe European Ceramic Society 1997, 17(11):1317-24; Kleinwechter, et al.,Journal of Materials Science 2002, 37(20): 4349-60). The synthesis isbased on gas phase reactions in a non-equilibrium plasma. In addition topreparing a wide range of nanoparticles, microwave plasma processingenables preparation of coated nanoparticles (Vollath, et al., Journal ofNanoparticle Research 1999, 1(2):235-42). Particles leaving the plasmazone are electrically charged so that their agglomeration is suppressedand high quality coating becomes possible. Recently, multipleplasma-based processes for synthesis of nanopowders were reviewed byVollath, Journal of Nanoparticle Research 2008 (in press)).

Metal oxide nanoparticles customized for reactive nanocompositematerials were developed in (Prakash, et al., Nano Letters 2005; 5(7):1357-60). Strong oxidizer nanoparticles (potassium permanganate) werecoated with a layer of a relatively mild oxidizer (iron oxide). Thecomposite oxidizer nanoparticles were synthesized by a new aerosolapproach in which the nonwetting interaction between iron oxide andmolten potassium permanganate aids the phase segregation of ananocomposite droplet into a core-shell structure. The iron oxidecoating thickness was varied to tune the reactivity of the product. Suchcore-shell engineered nanoparticles are promising for adjustment of thereaction rates in the nanocomposite energetic materials.

Nano-scaled oxides are also produced for applications in energeticmaterials using the sol-gel technique (Tillotson, et al., Journal ofNon-Crystalline Solids 2001; 285(1-3):338-45; Gash, et al., Chemistry ofMaterials 2001; 13(3):999-1007; Gash, et al., Journal of Non-CrystallineSolids 2001; 285(1-3):22-8), which can also produce nanocompositematerials.

A wet chemistry approach is generally useful for preparation ofnano-sized oxide particles suitable for energetic formulations. Forexample, a process for the preparation of nanosized tungsten oxideparticles is described by Perry, et al., Propellants, Explosives,Pyrotechnics 2004; 29(2):99-105. In that process, ammonium paratungstatewas dissolved in acid and the product, tungstic acid, was precipitatedby addition of distilled water. The product consisted of 7 nm thickplatelets of hydrated WO₃ with the plate length of about 100 nm. Suchparticles are useful for preparation of nanocomposite thermites.

Preparation of Reactive Nanocomposite Materials Powder Mixing

Given the availability of n-Al powders, most metal-based reactivenanomaterials are prepared by mixing such powders with additionalcomponents capable of highly exothermic reactions with aluminum.Literature data are available for many mixes of n-Al with nanosizedmetal oxide powders; including Fe₂O₃, MoO₃, CuO, Bi₂O₃, and others.These thermite compositions made of nanosized components were termed MIC(for metastable intermolecular composites or metastable interstitialcomposites) or superthermites (Perry, et al., Propellants, Explosives,Pyrotechnics 2004; 29(2):99-105; Danen, et al., 221st ACS NationalMeeting, San Diego, Calif., United States, Abstracts of Papers, 2001;Perry, et al., Journal of Applied Physics 2007;101(6):064313/1-064313/5; Son, et al., Proceedings of the 29thInternational Pyrotechnics Seminar Publisher Defence Science &Technology Organisation, Pyrotechnics Group; 2002, p. 871-7; Prakash, etal., Advanced Materials 2005; 17(7):900-3; Walter, et al., Journal ofPropulsion and Power 2007; 23(4): 645-50). A superthermite is commonlyprepared by ultrasonic mixing the nano-sized components in a bath ofhexane (Pantoya, et al., Propellants, Explosives, Pyrotechnics 2005; 30(1):53-62; Plantier, et al., Combustion and Flame 2005; 140:299-309;Sanders, et al., Journal of Propulsion and Power 2007; 23(4):707-14;Puszynski, et al., Journal of Propulsion and Power 2007; 23(4):698-706), isopropyl alcohol (Sanders, et al., Journal of Propulsion andPower 2007; 23(4):707-14; Puszynski, et al., Journal of Propulsion andPower 2007; 23(4): 698-706), or another liquid carrier, which issubsequently evaporated. The mixing is typically performed using highintensity ultrasonic actuators or commercially available ultrasonic celldisruptors. For liquids that can oxidize aluminum, such as isopropanol,the exposure time to the solvent is restricted. Assessment of thequality of mechanical mixing of nanopowders represents a majorchallenge. Such an assessment is highly desirable as a quality controlmeasure for the prepared nano-composite mixtures. A recent review (Wei,et al., Journal of Nanoparticle Research 2002; 4(1-2):21-41) showed thatmost current techniques are based on imaging of a relatively smallfraction of the prepared mixed sample, so that a large number ofmeasurements are required for a reliable assessment. Images presented inFIG. 10 show the variety of morphologies for nanocomposite MIC materialsobtained by powder mixing (Sanders, et al., Journal of Propulsion andPower 2007; 23(4):707-14). As clearly visible in FIG. 10, the issue ofmixing quality becomes especially critical when the morphologies ofnanopowders being mixed are very different, for example for the Al—MoO₃case shown in FIG. 10 a. Often, nano-oxide particles are not rounded orequiaxial but instead are shaped as rods or flakes. Also, aluminumflakes with nanometer thickness are often used and mixed with differentsize or shape oxide nanoparticles. A representative imaging of thesample is especially challenging because particles with large footprint(e.g., flakes) can shield smaller particles of the other components.

While relatively successful in laboratory evaluations, ultrasonic mixingof nanopowders is a process that is difficult to scale up. Processing oflarger sample batches inevitably leads to a lower quality of mixing.

Recently, thermite nanocomposites were produced by mixing the startingAl and Fe₂O₃ nano-powders using rapid expansion of a supercriticaldispersion (RESD) (Marioth, et al., International Annual Conference ofICT, 37^(th) (Energetic Materials). Publisher: Fraunhofer-Institut fuerChemische Technologie, 2006; p. 113/1-113/7). A better degree of mixingwas achieved compared to conventional ultrasonic mixing. In addition toproviding a higher quality of mixing, the RESD process is better suitedfor continuous operation than ultrasonic mixing. The main concern indeveloping and scaling up RESD for mixing reactive nanopowders appearsto be the safety of operation. In particular, the reactions between thesupercritical, or expanding CO₂, and nano-aluminum need to be prevented.In addition, the mixed nanocomposite is generated by RESD in anaerosolized form and thus there is the possibility of an aerosolexplosion.

Sol-Gel

An alternative to mechanical mixing of nanopowders was proposed(Tillotson, et al., Journal of Non-Crystalline Solids 2001;285(1-3):338-45; Gash, et al., Chemistry of Materials 2001;13(3):999-1007; Gash, et al., Journal of Non-Crystalline Solids 2001;285(1-3):22-8; Plantier, et al., Combustion and Flame 2005; 140:299-309;Gash, et al., Materials Research Society Symposium Proceedings 2003;800:55-66) describing sol-gel processing in which a nanocompositestructure is obtained with aluminum (or other metal) nanoparticlesresiding in the pores of a matrix made of the oxidizer material.Hydrated salts of metals serve as precursors and propylene oxide servesas a gelation agent. The process is carried out preferably in a polarprotic solvent and monolithic gels are produced. In order to preparereactive nanocomposites, metal powders are added just before thegelation while the solution is being stirred. The final step of removingthe pore fluid from the system is accomplished by either controlled slowevaporation or by supercritical extraction with CO₂. Respectively,xerogels or aerogels are produced. To produce aerogel, prior tosupercritical extraction, the liquid in the pores is replaced by CO₂ bya series of flush and drain cycles. Further functionalization of theoxide matrix is achieved using various silane additives (Clapsaddle, etal., Materials Research Society Symposium Proceedings 2003; 800:91-6).This approach is attractive as it naturally generates a very intimatemixing between the components. The disadvantages include: high porosityof the final composite makes it undesirable for some applications,restrictions on the types of materials that can be gelled, and adifficulty in scaling up the production rate—chiefly because of the needto introduce the metallic nanopowders in the stirred solution at a veryspecific stage of the gel preparation process.

Self Assembly

Recently, self-assembly approaches were considered to prepare reactivenanocomposite materials starting with nanosized aluminum powder andfunctionalized nanosized oxide particles (Gangopadhyay, et al., U.S.Pat. Appl. Publ. US 2007095445 A1 20070503, 2007; Subramaniam, et al.,Materials Research Society Symposium Proceedings 2006; 896:9-14;Apperson, et al., Applied Physics Letters 2007; 91, Article No. 243109;Mehendale, et al., Journal of Energetic Materials 2006; 24:341-60). Toproduce ordered assemblies, metal particles were arranged around theexterior surface area of oxide nanorods or in the ordered pore structureof the mesoporous oxidizer particles in composites. For example, theself-assembly in an Al—CuO system was achieved by initialfunctionalization of the CuO nanorods by applying a monofunctionalpolymer, poly(4)-vinyl pyridine (P4VP). The nanorods were prepared forthese experiments using the surfactant-templating method. Alnanoparticles adhere to functionalized nanorods and these “decorated”nanorods become ordered within the material (Subramaniam, et al.,Materials Research Society Symposium Proceedings 2006; 896:9-14;Apperson, et al., Applied Physics Letters 2007; 91, Article No. 243109).Such ordered structures are reported to produce higher flame speeds insmall-scale laboratory evaluation tests compared to nanocompositematerials with the same compositions conventionally mixed usingultrasonicated suspension. A conceptually similar approach was used toprepare self-assembled Al—Fe₂O₃ nanocomposite thermites (Mehendale, etal., Journal of Energetic Materials 2006; 24:341-60). A porous Fe₂O₃ wassynthesized using a micelles template-assisted sol-gel synthesis routeusing surfactants. The addition of surfactants during sol-gel synthesisgenerates an ordered pore structure. A reference sample of porous Fe₂O₃was also prepared by the same process but without the use ofsurfactants, so that the pore structure was not ordered. Both oxidizerswere mixed with n-Al and the combustion rates were compared to eachother. The ordered structure produced a higher flame speed. The orderednanocomposites are attractive as offering a better control over thematerial properties and potentially higher reaction rates in practicalapplications. The shortcomings of this approach include the high costsof custom-made oxides, the presence of functionalizing agents whichgenerally reduce the energy density of the energetic formulation, andthe inherently high porosity of the produced materials.

Layered Vapor Deposition

A distinct class of nanocomposite reactive materials is represented byreactive nanofoils. Well-controlled, nano-sized layers of materialscapable of highly exothermic reaction are coated on top of each otherusing vacuum deposition. One of the first descriptions of multilayeredAl—Ni films with the individual layer thickness varied from 60 to 300 nmis given by Ma, et al., Applied Physics Letter, 1990; 57(12):1262-4. Thefilms were produced at a pressure of 10⁻⁷ torr by alternate electronbeam evaporation of Al and Ni onto a photoresist coated glass slide. Thephotoresist was dissolved to obtain a free-standing film. The films werereported to react rapidly and sustain Self-propagating High temperatureSynthesis (SHS) reaction producing mixed phases of Al₃Ni, Al₃Ni₂, and Alwith a flame speed of about 4 m/s. This approach was systematicallydeveloped in a patent by Barbee and Weihs (Barbee, et al., U.S. Pat. No.5,538,795, 1996), followed by extensive research (Wang, et al., 2003Applied Physics Letters 83(19):3987-9; Wang J, et al., Journal ofApplied Physics 2004; 95(1):248-56; Duckham, et al., Journal of AppliedPhysics 2004; 96(4):2336-42; Gavens, et al., Journal of Applied Physics2000; 87(3):1255-63; Blobaum, et al., Journal of Applied Physics 2003;94(5):2915-22; Blobaum, et al., Journal of Applied Physics 2003;94(5):2923-9). Developed originally for joining applications (Wang, etal., 2003 Applied Physics Letters 83(19):3987-9; Wang, et al., Journalof Applied Physics 2004; 95(1):248-56; Duckham, et al., Journal ofApplied Physics 2004; 96(4):2336-42), reactive nanofoils attract moreand more interest as energetic components. Most work has been reportedfor Ni—Al nanofoils, for which many reaction details were studied,including the effect of partial annealing resulting in an increasedthickness of the partially reacted layer between bilayers of Al and Ni(Gavens, et al., Journal of Applied Physics 2000; 87(3):1255-63).Multilayer Al—Ni foils were prepared using magnetron sputtering, byrotating a water-cooled brass substrate over fixed Al and Ni guns. Thenumber of produced bilayers exceeded 4000. The thickness of individualbilayers varied from 25-80 nm. Substantial efforts were also made toprepare and characterize nanofoils of Al—CuO thermite (Blobaum, et al.,Journal of Applied Physics 2003; 94(5):2915-22; Blobaum, et al., Journalof Applied Physics 2003; 94(5):2923-9). CuO—Al multilayer foils weresimilarly prepared by magnetron sputtering performed in argon at 5mTorr. To avoid the reaction between CuO and Al, the sputter guns wereshielded to contain the plasma in a small volume above each target. Thesubstrate carousel was also water cooled to minimize mixing and reactingof the layers during deposition. The sputter-deposited CuO_(x) had thestructure of the mineral paramelaconite, Cu₄O₃. Each thermite bilayerwas 1 μm and the total foil thickness was 14 μm. Electron microscopyimages of the prepared thermite nanofoils are shown in FIG. 11.

Arrested Reactive Milling

Current literature describes only one “top-down” approach for preparingreactive nanocomposite materials where the nano-scaled structure isobtained by refining coarser starting materials. The nanocomposites areproduced using a technique similar to mechanical alloying, calledArrested Reactive Milling (ARM) (Dreizin, et al., US Patent Publication20060053970, the disclosure of which is incorporated by referenceherein; Schoenitz, et al., Materials Research Society Proceeding, 2004;800:AA2.6.1-AA2.6.6; Schoenitz, et al., Proceedings of The CombustionInstitute 2005; 30:2071-8; Umbrajkar, et al., Propellants, Explosives,Pyrotechnics 2007; 32(1):32-41; Dreizin, et al., International AnnualConference of ICT 2005, 36^(th) (Energetic Materials), p. 138/1-138/12;Umbrajkar, et al., Propellants Explosives and Pyrotechnics 2006:31(5):382-9; Umbrajkar, et al., Journal of Propulsion and Power 2008;24(2)192-8). The starting materials are mixtures of regular metal,metalloid and/or oxide powders. The sizes of starting powders are notcritical and using very fine or nanosized powders as starting materialsis, in fact, undesirable. In order to produce reactive nanocomposites,starting components are selected among materials capable of reactingexothermically. Metals and metal oxides (thermites) represent onepopular class of related compositions. Boron and metals such astitanium, zirconium, or hafnium forming respective borides representanother class of useful compositions. When powders of such materials aremixed and ball-milled, the exothermic reaction can be initiatedmechanically. Once initiated, the reaction becomes self-sustaining. Thereaction usually proceeds very rapidly resulting in substantialincreases in both the pressure and temperature within the millingvessel. Reactive nanocomposites are produced when the milling process isinterrupted (or arrested, hence ARM) just before the self-sustainingreaction is mechanically triggered. For small scale samples, preliminaryexperiments are used to establish the time when the self-sustainingreaction is triggered. For larger scale samples, the milling conditionsand the time of milling are predicted using numerical modeling of energytransfer between the milling media and the powder (Ward, et al., ActaMaterialia 2005; 53:2909-18).

ARM leads to the formation of fully dense, micron-sized compositeparticles with nanoscaled structural features. Each particle is athree-dimensional composite of starting materials as opposed tohomogenized or chemically bonded compounds thereof. The milling time atwhich the reaction is mechanically triggered effectively sets a limit tothe spatial scale on which the components are mixed. This time limit canbe influenced by the specific milling parameters chosen—the powder batchsize, the mass ratio of powder sample to that of the milling media, theprocessing temperature, and the use of process control agents.Collisions between the milling media subject the milled powder totransient pressures of up to 5 GPa (Suryanarayana, Progress in MaterialsScience 2001; 46(1-2):1-184), individual particles have therefore neartheoretical maximum density (TMD).

ARM processing is very flexible and versatile. It does not have the manylimitations of chemical or vacuum condensation techniques, which can beused only with selected compositions. It has been observed thatessentially any combination of reactive materials can be processed byARM to prepare a nanocomposite reactive powder. Table 4 lists allcompositions prepared by ARM to date including multiple thermites andmetal-metalloid systems. The process is readily scalable andinexpensive. One of the important limitations of the ARM processing isthe inevitable presence of a fraction of reacted material in theprepared nanocomposite powder. Such reacted material forms in relativelysmall quantities during the processing when a reaction between thecomponents is locally triggered mechanically but is not self-sustainedwithin the sample. As a result, the small quantities of the reactionproducts are redistributed and homogenized within the sample that iscontinued to be ball milled. The presence of the partial reactionproducts can be minimized by adjusting the milling conditions andparameters (Umbrajkar, et al., Propellants Explosives and Pyrotechnics2006: 31(5):382-9; Umbrajkar, et al., Journal of Propulsion and Power2008; 24(2)192-8). The safety of the ball mill operation must beconsidered when preparing reactive nanocomposite materials by ARM.Specifically, the self-sustaining reaction between the reactivecomponents must be prevented to avoid the damage to the processingequipment and facility.

TABLE 4 Reactive nanomaterials prepared by ARM to date Oxidizer FuelFe₂O₃ MoO₃ CuO Bi₂O₃ WO₃ SrO₂ NaNO₃ Nanocomposite Thermites Al x x* x* xx x x** Mg x x x Al_(0.5)Mg_(0.5) x MgH₂ x x Si x x x Zr x x x x 2B +Ti*** x** 2B + Zr*** x** Reactive Metal-Metalloid composites B Reactivemetals: Ti, Zr, Hf Si Reactive metal: Ti Nanostructured Al-based alloysAl Alloying components: W, Fe, Hf, Mg, MgH₂, Ti, Li, Zr, C, I, Zn*Metal-rich nanocomposites also have been synthesized **Metal-leannanocomposites also have been synthesized ***Nanocomposite powder usedas component for compound nanocomposite

A characteristic cross-section of a particle of an Al—CuO nanocompositeprepared by ARM is shown in FIG. 12. The inclusions of CuO in Al matrixvary in size from 1 μm down to less than 50 nm. It is important to notethat inclusions of one component (e.g., CuO for the example shown inFIG. 12) are fully embedded into the matrix of the other component(e.g., Al). Therefore, the entire Al/CuO interface area will participatein the exothermic heterogeneous reaction upon thermal (or other)initiation of such a material. This would not be the case for a materialprepared as a mixture of nanopowders in which the reactive particlesonly have direct contact over a relatively small portion of the totalsurface area of each nanosized component. The propagation mechanisms forthe reactions in fully dense nanocomposites prepared by ARM and in thehighly porous mixtures of nanopowders with identical bulk chemicalcompositions may be entirely different. The pores may play a criticalrole in promoting the pressure-driven reaction propagation, while aslower, thermal reaction propagation mechanism is expected for the fullydense nanocomposites.

Reactive milling has also been investigated in Russia to produceenergetic compositions refined on the nanoscale termed MechanicallyActivated Energy Composites (MAEC) (Dolgoborodov, et al., KhimicheskayaFizika 2004; 23(9):85-9; Dolgoborodov, et al., JETP Letters 2005;81(7):311-4; Dolgoborodov, et al., Khimicheskaya Fizika 2007;26(12):40-5). Most of the effort focused on metal-Teflon compounds(Dolgoborodov, et al., Khimicheskaya Fizika 2004; 23(9):85-9;Dolgoborodov, et al., JETP Letters 2005; 81(7):311-4). Al, Mg, Ti, andZr powders were used while major experiments focused on Al- and Mg-basedcomposites (Dolgoborodov, et al., Khimicheskaya Fizika 2007;26(12):40-5). Similar to the ARM approach, the milling conditions in avibratory mill are selected to avoid or minimize the reaction betweenthe components while achieving maximum degree of homogenization betweenthe starting materials.

Materials Properties

Knowledge of material properties is essential for understanding theirreaction mechanisms, for prediction of their performance in energeticformulations, and for designing practical systems and componentsemploying such materials. Some of the material properties commonly usedfor micron-sized reactive powders and structures must be redefined formaterials with nano-scale features. For example, many reaction featuresfor conventional metal fuels are affected by such thermodynamicproperties as melting and boiling point of the metals and theirrespective oxides. However, for material domains of nanometer dimension,the structure and phase stability are strongly affected by the surfaceenergy. As a result, melting can occur at different temperatures fordomains (particles or crystallites) of different sizes and may not beobserved at all if the domains are sufficiently small, e.g., see(Tanaka, et al., Zeitschrift fuer Metalikunde/Materials Research andAdvanced Techniques 2001; 92(5):467-72; Goldstein, et al., Science 1992;256(5062):1425-7; Lai, et al., Physical Review Letters 1996;77(1):99-102; Allen, et al., Thin Solid Films 1986; 144(2):297-308). Theenthalpy of formation of respective nanodomains and their combustionenthalpies will also be altered compared to the coarser components withthe same elemental compositions. Effects of surface energy on theboiling temperature and on the equilibrium vapor pressure as a functionof temperature can also be significant (Farrell, et al., Journal ofVacuum Science & Technology, B: Microelectronics and NanometerStructures—Processing, Measurement, and Phenomena 2007; 25(4): 1441-7).Similarly to thermodynamic properties, mechanical properties ofnano-sized particles or surface layers coating such particles cannot bedescribed using conventional characteristics for bulk materials (Misra,et al., Advanced Engineering Materials 2001; 3(4):217-22).

Particle Size Distributions, Surface Morphology, and Active MetalContent

The sizes of the nano-domains and the nanoparticles, are of primaryimportance in determining the rates of heterogeneous reactions commonlyleading to ignition of such materials. Similarly, the particle sizedistributions and related specific surface values of nano-compositematerials affect their aging characteristics, i.e., changes in the oxidelayer thickness, partial reaction between components, etc. Particlesizes must be known to design appropriate material handling and mixingtechniques. Because of the polydisperse nature of nanopowders andnanodomains observed in reactive nanocomposites, the quantitativedescriptions of the respective particle size distributions are difficultto obtain. Agglomeration that can be very significant for nanopowdersmakes quantitative description of the particle distributions even moredifficult. It should be noted that the combustion performance may bequite different for the powders with the same specified prime diametersor even with similar size distributions for the primary particles if oneof the powders is much more agglomerated than another. The agglomerationof nanopowders is unavoidable (Singhal, et al., Nanostructured Materials1999, 11(4):545-52), while its detailed mechanisms and their correlationwith other powder properties are poorly understood.

The problem of quantifying the particle size distribution for thenano-composite materials becomes even more challenging considering thatat least two nano-scaled phases are present. For the simplest case ofaluminum nanopowder, the second phase is the passivating oxide (or anyother) surface layer. For the more complex superthermites,intermetallic, or metal-metalloid composites, additional phases includevarious oxides, metals, alloys, and metalloids.

Various particle shapes introduce yet another layer of complexity in thequantitative particle size distribution measurements. Most particlesizing techniques implicitly assume that the particles are spherical inshape. Depending on the preparation technique, this assumption may ormay not hold true for different aluminum nanopowders. The assumption ofthe spherical particle shape is most likely to be incorrect for oxideand metalloid nanoparticles. An approach for describing particledimensions and shapes for non-spherical and highly agglomeratedparticles based on analysis of the powder fractal dimensions has beendeveloped and used extensively for many agglomerated powders andaerosols (Kindratenko, et al., Environ. Sci. Technol. 1994, 28:2197-202). Most commonly, textural and density (or structural) fractaldimensions of the aerosol particles are considered (Colbeck, et al., J.Aerosol Sci. 1997, 28:715-23; Colbeck, et al., J. Aerosol Sci. 1997,28:715-23). This approach may prove to be useful for characterization ofmany non-spherical and agglomerated reactive nanopowders.

Substantial efforts were made to characterize aluminum nanopowders whichserve as the most popular component of reactive nanomaterials. Withadvances in electron microscopy, imaging of the nanoparticle samplesbecame a common practice. Respective SEM or TEM images can be readilyused for straightforward, although labor-intensive, size classificationof the observed particles, e.g., (Ermoline, et al., Nanotechnology 2002;13:638-43). In order to be representative, such measurements mustconsider a large number of particles. It is also desired that theparticle sizing be performed in different locations of the sampleprepared for the electron microscopy. In addition to the directmeasurement of the particle size distribution, electron microscopyprovides very important information about the particle shapes, thethickness of the oxide layer (high resolution TEM), and about theparticle surface morphology. This information is important forinterpreting the results of various other particle sizing techniquesbased on light scattering, surface areas by gas absorption, etc.

The most common characterization of the particle size is obtained fromthe specific surface measurements. Usually, a BET(Brunauer-Emmett-Teller) measurement of gas adsorption is employed,e.g., (Puszynski, et al., Materials Research Society SymposiumProceedings 2006; 896:147-58; Moore, et al., Journal of Propulsion andPower 2007; 23(1):181-5; Prentice, et al., J. Phys. Chem. B 2005;109:20180-5) to obtain a single number characterizing the dimension ofthe specific nanopowder. The “BET diameter” is often introduced byassuming that the measured specific surface of a powder is equivalent tothat produced by monodisperse spherical particles. For many applicationsthis represents the single most important characteristic of ananomaterial. A similar, “one number” assessment of the particle sizecan be obtained for nanopowders and for fully dense nanocompositematerials using broadening of the x-ray diffraction line and the wellknown Scherrer formula (Patterson, Physical Review 1939; 56(10):978-82). Modifications of this approach have also been discussed in theliterature to obtain a more accurate assessment of the nano-domaindimension (Hall, et al., Journal of Applied Crystallography 2000; 33(6):1335-41). However, for many energetic materials, the width and shape ofthe particle size distribution are as important as any specific weightedaverage diameter, so that additional measurements are needed to obtainreliable particle size distributions.

Few commercial devices are available for sizing nanopowders. Submicronparticle sizes can be quantified using low-angle laser light scatteringand several commercial instruments utilizing this technique areavailable. These instruments are generally designed for sizecharacterization of micron-sized powders and their range of measurementsis extended to include particles as small as 40 nm with specificalgorithms for processing the measured scattered laser emission. Themain advantage of this type of measurement is that a powder withrelatively broad particle size distribution can be characterized.However, for accurate measurements the optical properties of thematerial surfaces need to be known. Such properties are not wellestablished for many materials, especially, for the nano-sizedparticles, for which, the material properties are expected tosubstantially differ from those of the respective bulk materials.Another group of devices uses photon correlation spectroscopy, whichdetermines particle size by measuring the rate of fluctuations in laserlight intensity scattered by particles as they diffuse through a fluid.The measurement needs to be processed assuming a specific shape of thesize distribution function, so that generally the mean particle size andthe width of the particle size distribution can be quantified. Forsuccessful measurement, the powder needs to have a relatively narrowsize distribution and the measurements become less and less useful asthe width of the particle size distribution increases.

Currently, new approaches are being actively developed forcharacterization of the particle sizes of reactive nanomaterials. In onerelated approach, the particle size distributions are determined usingthermo-gravimetric analysis (TGA) (Johnson, et al., Journal ofPropulsion and Power 2007; 23(4):669-82). TGA measurement enablesidentification of the active metal content, e.g., the amount ofun-oxidized aluminum for the Al nanopowder. In addition, the presence ofvolatile impurities can be detected and particle size distributions canbe obtained. Interpreting the TGA curve, by considering the effect ofparticle oxidation of a pre-selected particle size, produces a measureof the primary particle size in the powder tested, similar to the gasabsorption measurement. Comparison of TGA, BET, and SEM measurements ofparticle sizes, with the XRD analysis yielding a crystallite size, ispresented by Johnson, et al., Journal of Propulsion and Power 2007;23(4):669-82 for 12 different nano-aluminum samples. The resultsgenerally compare well to one another. However, for samples with thecoarser particles implied by the TGA and SEM measurements, XRD did notshow an appreciable increase in the crystallite sizes. In order toobtain particle size distributions, a curve-fit procedure is used torepresent the experimental TGA curve as a superposition of “basiccurves” that ideally represent the mono-modal powder fractions. Thebasic curves are selected based on preliminary information about thespecific nanopowder, e.g., obtained from SEM images. The mainassumptions made were the shape of the particles (spherical), thethickness of the initial oxide layer, and the oxide density. Differentalumina polymorphs have substantially different densities (Levin, etal., J. Am. Ceram. Soc. 1998; 81(8):1995-2012), so using an incorrectdensity for the starting “natural” alumina layer can cause a large errorin the final particle size distribution. The TGA-based method was shownto be well suited for monitoring samples for the presence of larger,0.5-5 μm diameter particles but ineffective for quantitative sizedistributions of powders containing particles smaller than 100 nm(Johnson, et al., Journal of Propulsion and Power 2007; 23(4):669-82).The main problem identified by Johnson, et al., Journal of Propulsionand Power 2007; 23(4):669-82 was the inconsistency in oxidation ofvarious nanopowders at lower temperatures. This inconsistency wasattributed to possible impurities in some of the nanopowders. Inaddition, particle coalescence during or after melting can occurresulting in the change of the particle sizes and respective differencesin the oxidation behavior. Particles with dimensions greater than 10 μmimpose another limitation because such particles may remain incompletelyoxidized even at the highest temperatures (typically, 1500° C.) achievedby TGA.

Another particle size measuring approach, suitable for many nanopowdersor nanocomposite materials, involves small-angle X-ray and/or neutronscattering (SAXS, SANS) (Trunov, et al. Journal of Physical Chemistry B,2006; 110(26): 13094-9; Mang, et al., Journal of Materials Research2007; 22(7):1907-20; Borchert, et al., Langmuir 2005; 21(5):1931-6). Inrefs. (Mang, et al., Journal of Materials Research 2007; 22(7): 1907-20)and (Borchert, et al., Langmuir 2005; 21(5):1931-6), the results of SAXSand SANS measurements were shown to compare well to the results ofelectron microscopy, TGA, BET, and X-ray diffraction measurements. Anexample of such a comparison for SEM, SANS and SAXS results is shown inFIG. 13 (Mang, et al., Journal of Materials Research 2007;22(7):1907-20). A detailed discussion of the underlying theoreticalapproach, as presented by Mang, et al., Journal of Materials Research2007; 22(7): 1907-20, showing that specific information about theparticle shape and morphology is needed in order to meaningfully processthe scattering measurements. This information can be gained fromdetailed electron microscopy studies complementing the x-ray and neutronscattering measurements. Because x-rays and neutrons interact withmatter differently, it is possible not only to quantify the particlesize distributions, but also characterize the features of the compositeparticles. For example, the thickness of the oxide layers was evaluatedwith good accuracy based on the processed scattered intensity data(Mang, et al., Journal of Materials Research 2007; 22(7): 1907-20). Inorder to transform the scattering results into a size distribution, aspecific model for the size distribution function must be assumed. Inmany cases (Trunov, et al. Journal of Physical Chemistry B, 2006;110(26): 13094-9; Borchert, et al., Langmuir 2005; 21(5):1931-6), theassumption of the lognormal distribution is well justified.

One of the more unconventional approaches used to size classify reactivenanopowders relied on a differential mobility analyzer (Jouet, et al.,Chemistry of Materials 2005; 17(11):2987-96) well suited for measuringparticle sizes for very fine airborne particles. Similar to the TGAtechnique, the assumptions about the thickness and density of theinitial oxide layer (or impurities) affect the output substantially.Also well-suited for the nano-sized airborne particles is a techniquedescribed by Mukherjee, et al., Aerosol Science 2006; 37:677-695 andusing quantitative laser-induced breakdown spectroscopy (LIBS).Qualitative LIBS was shown to be effective in finding the degree ofoxidation of n-Al particles processed at different temperatures.

Generic SEM and TEM imaging techniques are invaluable forcharacterization of particle sizes, shapes, and surface morphology. Highresolution TEM is successfully used to characterize the oxide coatingspresent on aluminum particles exposed to oxidizing environments. Theinformation about thickness, crystallinity, microstructure, andhomogeneity of the oxide layer is of critical importance forunderstanding the mechanisms of oxidation of metal nanoparticles. Inturn, the mechanisms of oxidation affect both ignition and agingkinetics of many nanocomposite reactive materials. Most of the reportsagree that the thickness of the oxide layer on the surface of aluminumparticles is essentially independent of the particle size and isgenerally in the 2.5-3 nm range.

It is currently well established that the natural oxide layer onaluminum particles is amorphous. However, the images presented in FIG.14 also show that the oxide coating is not homogeneous and includesprecursors of the growing g-Al₂O₃ crystallite sheets. As shown in FIG.14, these mismatching crystalline sheets can be exfoliated from the mainamorphous film.

Characterization of composite materials is more difficult compared toelemental metal nanoparticles and much less progress has been made. Forthe materials prepared using starting nanosized powders, e.g. bymechanical mixing or sol-gel synthesis, the measure of the reactivesurface is commonly obtained from the starting particle sizes. For thefully dense, nanocomposite materials, scanning electron microscopy ofthe cross-sectioned materials and XRD measurements of the crystallitesizes produce the data about the size distributions of the nanodomains,e.g., (Umbrajkar, et al., Propellants Explosives and Pyrotechnics 2006:31 (5):382-9).

Finding the active (or metallic) Al content in powders is closelyconnected to the correct identification of the particle size, as alreadymentioned above. High resolution TEM images showing the thickness of thealumina layer combined with the data on the particle size distributioncan be used to estimate either the volumetric or gravimetric fraction ofpure Al in the powder. The active aluminum content can also bedetermined from TGA results by measuring the degree of powder oxidationupon heating. The above approaches all rely on assumptions about thedensity, structure, and homogeneity of the initial oxide layer. Yet, ithas been reported that such layers can be porous, partially hydrated,somewhat non-uniform in thickness, or can contain some adsorbed gases(Ramaswamy, et al., Energetic Materials 2005; 23:1-25; Fedotova, et al.,36th Int. Annual Conference of ICT combined with the 32nd Int.Pyrotechnics Seminar, Karlsruhe, Germany, Jun. 28-Jul. 1, 2005, p.147/1), all of which would affect the accuracy of the calculatedavailable active metal content. The situation is complicated further formaterials that are processed to produce specific passivating layers.Techniques that enable finding the active aluminum content for suchcases include quantitative measurements of the gas evolved upon basehydrolysis (Johnson, et al., Journal of Propulsion and Power 2007;23(4):669-82), chemical analysis using induced coupled plasma (ICP)emission to determine the Al/Al₂O₃ ratio (Rufino, et al., ActaMaterialia 2007; 55:2815-27), or various wet chemistry techniques, suchas permanganate (Fedotova T D, et al., Propellants, Explos. Pyrotech.2000; 25:325-32) or cerimetric (cerium titration) (Fedotova T D, et al.,Propellants, Explosives, Pyrotechnics 2007; 32(2): 160-4) methods. Anassessment of the active aluminum content can also be made using oxygenbomb calorimetry (Kwon Y-S, et al., Applied Surface Science 2003; 211;57-67).

Thermodynamics of Nano-Scaled Components

At the spatial scale of nanoparticles and nanodomains, thermodynamicproperties of materials are altered. This includes changes in themelting point and latent heat of melting, which are of particularsignificance for reactive nanomaterials. In addition, phasetransformations, such as polymorphic transitions between differentaluminum oxide phases, are also affected by the reduced dimensions ofthe oxide films. Properties of other oxides or metals in othernanomaterials are affected similarly, but discussed much less in theliterature.

The changes in the melting point for materials reduced to nano-sizedparticles or inclusions are extensively discussed in literature, e.g.,Alcoutlabi, et al., J. Phys. Condens. Matter 2005; 17:R461-R524. Thisreview of recent results showed that the Gibbs-Thomson model widely usedto explain the depression of the melting point, T_(m), is deficient atthe nano-scale and further work needs to be done to fully capture sizeeffects on melting behavior. In this review, the issue of depressedmelting point is discussed in the context of its particular importancefor reactive nanoparticles because melting is expected to substantiallyaffect both ignition and combustion behaviors. Melting and oxidation ofaluminum nanoparticles was recently systematically addressed by Trunov,et al. Journal of Physical Chemistry B, 2006; 110(26):13094-9 andRufino, et al., Acta Materialia 2007; 55:2815-27; Sun, et al.Thermochimica Acta 2007; 463(1-2):32-40) using differential scanningcalorimetry. The main advantage of this approach is that the actualparticle size distribution for each specific powder sample was obtainedand used instead of the more common weighted average particle size orbulk value of the powder. The experimental particle size distributionswere obtained using SAXS (Mang, et al., Journal of Materials Research2007; 22(7): 1907-20; Borchert, et al., Langmuir 2005; 21(5):1931-6).Two lognormal distributions were used to fit the experimental SAXS datafor each analyzed nanopowder. While the shape of the size distributionswas assumed, the mean modal diameters and widths of both lognormaldistributions were varied to obtain the best fit with the SAXSmeasurement. The experimental particle size distribution functions wereused to predict the differential scanning calorimetry (DSC) meltingcurves for different powder samples according to different modelsproposed for the melting depression as a function of particle size.These melting point models will be discussed briefly below.

A model describing the melting point depression and referenced inseveral recent papers dealing with aluminum nanopowders used incombustion systems, e.g. Hunt, et al., Acta Materialia 2004; 52(11):3183-91; Hunt, et al., Intermetallics 2006; 14(6):620-9; Granier, etal., Combustion and Flame 2004; 138(4):373-83, was developed decades agoby Reiss and Wilson Reiss, et al., J. Colloid Sci. 1948, 3:551-61. Themodel describes the melting point, T_(m), as a function of the particlediameter, D_(p), and the oxide film thickness h_(ox), as:

$\begin{matrix}{T_{m} = {T_{b}\left\lbrack {1 - \frac{4\sigma_{sl}}{H_{b}{\rho_{Al}\left( {D_{p} - {2h_{ox}}} \right)}}} \right\rbrack}} & (1)\end{matrix}$

where T_(b) is the melting temperature of bulk aluminum (933.47 K or660.32° C.), H_(b) is the enthalpy of fusion of bulk aluminum, and4σ_(sl) is the interfacial surface tension between the solid and theliquid. The difference in the molar volumes between solid and liquidaluminum was neglected.

More recently, a theoretical model of melting for nanocrystalline metalpowder was developed by Jiang et al., (Reiss, et al., J. Colloid Sci.1948, 3:551-61; Jiang, et al., Materials Letters 2002; 56:1019-21;Liang, et al., Physica B 2003; 334:49-53; Zhao, et al., Solid StateCommunications 2004; 130:37-9). The melting point was suggested todepend on the diameter of aluminum nano-crystals, D, often assumed to beequal to the diameter of the aluminum core for the oxide-coated aluminumnanoparticles, as:

$\begin{matrix}{T_{m} = {T_{b}{\exp\left( {{- \frac{2H_{b}}{3{RT}_{b}}}\frac{1}{\left( {{{D/6}l} - 1} \right)}} \right)}}} & (2)\end{matrix}$

where R is the universal gas constant and l is the length of the Al—Alatomic bond. It was further suggested that for nanoparticles, the latentheat of melting, H_(m), depends on the particle diameter as:

$\begin{matrix}{H_{m} = {H_{b}{{\exp\left( {{- \frac{2H_{b}}{3{RT}_{b}}}\frac{1}{\left( {{{D/6}l} - 1} \right)}} \right)}\left\lbrack {1 - \frac{1}{\left( {{{D/6}l} - 1} \right)}} \right\rbrack}}} & (3)\end{matrix}$

Processing the experimental results reported by Eckert et al., (Eckert,et al., Nanostructured Materials 1993; 2:407-13) for composite materialscontaining nanosized aluminum inclusions produces phenomenologicaldependencies for both the melting point (K) and latent heat of melting(kJ/mol) as functions of the aluminum core diameter (nm):

$\begin{matrix}{T_{m} = {977.4 - \frac{1920}{D}}} & (4) \\{H_{m} = {14.705 - \frac{177.49}{D}}} & (5)\end{matrix}$

Some experimental data limit the applicability of Eqs. (4, 5) toparticles with diameters in the range of 12 nm<D<43 nm. For largerparticles, T_(m)=T_(b) and H_(m)=H_(b). For particles with the metalcore smaller than 12 nm, the latent heat of melting is reported to benegligible (Eckert, et al., Nanostructured Materials 1993; 2:407-13).Free aluminum nanoparticles with sizes less than 20 nm are not currentlyavailable, but the approach described by Sun, et al. Thermochimica Acta2007; 463(1-2):32-40 enables direct experimental study of melting insuch particles obtained by controlled oxidation of coarser startingpowders.

The three different melting models introduced above, were used with theexperimental PSD to quantitatively predict the DSC signals expected formelting of different commercial nanopowder samples. Each calculation wascompared to a specific DSC run. Additional corrections for possiblesample aging were made as described in detail by Trunov, et al. Journalof Physical Chemistry B, 2006; 110(26):13094-9. Equations (1), (2), and(4) give the functional dependence of the melting temperature on theparticle diameter. These equations were converted into equations thatgive the functional dependence of the aluminum core diameter on themelting temperature, D=D(T_(m)). The converted equations, and theirtemperature derivatives

$\frac{D}{T},$

were used to predict the DSC signal directly. The DSC signal, {dot over(Q)}, predicted for each specific temperature, T, and each specificheating rate, b, was calculated as:

$\begin{matrix}{{\overset{.}{Q} = {M_{s}{P(D)}\frac{\pi}{6}D^{3}{H_{m}(D)}\frac{D}{T}\beta}},} & (6)\end{matrix}$

where P(D) is the normalized frequency function, of aluminumnanoparticle distributions obtained from SAXS measurements and M_(s) isthe normalization parameter for P(D) accounting for the aluminum metalmass in the analyzed sample.

Comparisons of the experimental DSC melting curves with the predictedDSC curves from these models, are presented in FIG. 15. There is noclose agreement between the experimental DSC curve and any of meltingmodel predictions; however, the overall shape of the melting endothermwas predicted by all the models. In particular, it is interesting thatthe experimental endotherms have at least two peaks and this overallshape is generally predicted considering the bimodal size distributionsobtained from SAXS. According to Eqs. (4, 5) (Eckert, et al.,Nanostructured Materials 1993; 2:407-13), only a fraction of the powderis predicted to melt below T_(b)=660.32° C. The rest of the powder, asnoted in FIG. 15, is expected to melt at a constant temperature ofT_(b), so that respective calculations for the melting endotherms couldnot be performed and are not shown in FIG. 15.

Trunov, et al. Journal of Physical Chemistry B, 2006; 110(26): 13094-9found that the shapes of the predicted curves are very sensitive to thespecific type of the particle size distribution, e.g., bimodal vs.single mode lognormal distribution. The melting curves can furtherdepend on the specific surface morphologies of the used nanoparticlesthat could affect the sizes of melting nano-domains. Independentexperiments (Trunov, et al. Journal of Physical Chemistry B, 2006;110(26):13094-9; Rufino, et al., Acta Materialia 2007; 55:2815-27; Sun,et al. Thermochimica Acta 2007; 463(1-2):32-40) confirm that the meltingof aluminum nanopowders starts at a lower temperature than the meltingpoint of bulk aluminum; however, existing models describe the effect ofthe particle size on the melting point depression only qualitatively.Detailed analysis of the particle size distributions and surfacemorphology is needed for quantitative verification of any relatedmodels.

The thermodynamic parameters of oxides or other nano-scale materialcomponents used in reactive nanocomposite materials have not beendiscussed in the literature dealing with reactive materials and theirapplications. At the same time, reducing the crystallite or particlesize to the nano-scale certainly affects the thermodynamic propertiesand the stability of oxides, hydroxides, and other related compounds,e.g., see reviews (Navrotsky, Mineralogy and Geochemistry 2001;44:73-103; Navrotsky, Geochemical Transactions 2003:4(6):34-7). The lackof attention to such effects in the community dealing with reactive andenergetic materials is most likely due to fact that the reactionenthalpies in the systems of interest (e.g., thermites) are much greaterthan the anticipated effects of fine particle sizes on the enthalpy offormation or surface energy. On the other hand, often the reaction ratesin nanocomposite materials are limited by the diffusion of fuel and/oroxidizer through the growing or decomposing oxide layers. Thus, theproperties of such layers determine the diffusion and respectivereaction rates. A relevant example is the recently developed aluminumignition model in which the rate of aluminum oxidation is calculated asa function of oxide layer thickness and, most significantly, thepolymorphic modifications in the aluminum oxide layer present on theparticle surface (Trunov, et al. Combustion and Flame 2005; 140(4):310-8; Trunov, et al. Combustion Theory and Modelling 2006;10(4):603-23). Polymorphic transformations occur in alumina as afunction of both temperature and thickness of the oxide film (Jeurgens,et al., Phys. Rev. B 2000; 62(7):4707-19), while the rates of diffusionthrough different alumina polymorphs differ substantially. In addition,polymorphic phase changes in alumina are accompanied by substantialdensity change, so that the thickness and continuity of the oxide layerand thus the oxidation rate can be dramatically affected by suchtransitions, even though the transition energy is negligible compared tothat of aluminum oxidation heat release. In another relevant case, manyimportant thermite reactions occur through formation of multipleintermediate phases. Likewise, for the 2Al+MoO₃→Al₂O₃+Mo reaction tooccur in the solid state (at relatively low temperatures), based on theMo—O phase diagram, molybdenum oxide is likely to form a number ofoxides, MoO₃→Mo₉O₂₆→Mo₄O₁₁→MoO₂ before being reduced to metallic Mo.When the initial MoO₃ particle or inclusion is of the nano-dimension,the stability of different produced suboxides can be affected and,respectively, the sequence of the phase changes can be altered comparedto that implied by the known phase diagrams developed for bulkmaterials. Therefore, the reaction mechanism in such thermite systemsmay depend on the dimensions of the initial oxide particles and layers.Detailed studies of the stability of and phase transitions in variousnano-scaled oxides used in reactive nanocomposite materials are thusneeded for the mechanistic understanding and modeling of theheterogeneous reactions in such materials.

Exothermic Heterogeneous Reactions

The exothermic heterogeneous reactions both drive ignition and affectaging of reactive nanomaterials. Among such reactions, oxidation ofnano-aluminum is the most fundamental and occurs in all such materialscontaining aluminum. This oxidation process was recently studiedextensively and its mechanisms continue to be the subject ofconsiderable debate.

Aluminum Oxidation in Gaseous Oxidizers

The primary experimental technique used for studies of oxidation of n-Alin oxygen-based, vapor phase oxidizers is thermal analysis. TGA curvesshow the sample weight increase as a result of oxidation andsimultaneous DSC measurements can be used to quantify the correspondingheat effects. TGA and DSC studies of oxidation of various nano-sizedaluminum powders are reported in many papers, including (Johnson, etal., Journal of Propulsion and Power 2007; 23(4):669-82; Trunov, et al.Journal of Physical Chemistry B, 2006; 110(26):13094-9; Rufino, et al.,Acta Materialia 2007; 55:2815-27; Mench, et al., Combust. Sci. Technol.1998; 135:269-92; Jones, et al., Journal of Thermal Analysis andCalorimetry 2000; 61(3):805-18; Sun, et al., Thermochimica Acta 2006;444(2): 117-27) and others. The experimental results are generallyconsistent between themselves and show that for particles of about 100nm and finer, the majority of oxidation occurs at temperatures below thebulk aluminum melting point. This result was obtained with variousaluminum nanopowders, using constant heating rates (most authors) andisothermal experiments (Jones, et al., Journal of Thermal Analysis andCalorimetry 2000; 61(3):805-18). Examples of combined DSC curvesmeasured in inert environments and TGA curves measured for the samepowders in oxygenated gas are shown in FIG. 16 (Trunov, et al. Journalof Physical Chemistry B, 2006; 110(26): 13094-9) (all measurements areat the heating rate of 5° C./min). The first well-defined oxidation stepoccurs between 500 and 600° C. For nano-sized aluminum powders, thisstep results in a substantial weight increase. The onsets of the meltingendotherms for all powders shown in FIG. 16 are clearly observed at ahigher temperature. Interestingly, the first oxidation step for themicron-sized aluminum powders occurs at the same temperatures as fornanopowders, resulting, however, in a very small mass increase, e.g.,(Trunov, et al. Combustion and Flame 2005; 140(4): 310-8; Trunov, et al.Combustion Theory and Modelling 2006; 10(4):603-23; Johnson, et al.,Journal of Propulsion and Power 2007; 23(4):669-82). Furthermore, theoxidation kinetics identified by Trunov, et al. Combustion and Flame2005; 140(4): 310-8; Trunov, et al. Combustion Theory and Modelling2006; 10(4):603-23 for different steps observed for micron-sizedaluminum powders explains successfully the oxidation of nanosizedpowders (Trunov, et al. Journal of Physical Chemistry B, 2006;110(26):13094-9).

The TGA results presented above are generally inconsistent with a set ofdata obtained using single particle mass spectrometry (SPMS) (Park, etal., J. Physical Chemistry B. 2005; 109(15):7290-9; Rai, et al., J.Phys. Chem. B 2004; 108:14793-5). Unlike TGA experiments, SPMSmeasurements are performed on individual nanoparticles. Thenanoparticles are either generated within the experimental setup bylaser or arc ablation of an aluminum target or from commercial powdersfed into the apparatus from a methanol suspension. The particles arepassed through a furnace with a section heated to a target temperatureand then fed into the SPMS analyzer. Characteristically, the heatedsection is 30 cm long and the typical exposure time is 1 s. In theanalyzer, the particles are fed into a powerful laser beam ensuring thecomplete ablation and ionization of all particles. The produced ions aresampled with a linear time of flight mass spectrometer. Thus, theabundances of aluminum and oxygen atoms are obtained and the differencesbetween particles with different degrees of oxidation are identified.

The SPMS results detect substantial oxidation only above the meltingpoint of aluminum. These results were found to be in quantitativeagreement with the hot-stage TEM observations also reported by Rai, etal., J. Phys. Chem. B 2004; 108:14793-5. The TEM imaging showedpronounced changes in morphology and evidence of aluminum leaking outfrom the oxide shell once the aluminum melting point was exceeded. Park,et al., J. Physical Chemistry B. 2005; 109(15):7290-9 and Rai, et al.,J. Phys. Chem. B 2004; 108:14793-5 argue that their results are moreaccurate than TGA measurements (which they also performed obtainingresults consistent with other published TGA experiments (Park, et al.,J. Physical Chemistry B. 2005; 109(15):7290-9)). The main proposedreason for the improved accuracy is that single, non-interactingparticles are studied in the SPMS experimental configuration whileparticle interaction within the TGA sample could cause heating of thesample above the controlled TGA furnace temperature.

Review of multiple references describing similar TGA measurements foroxidation of a broad range of aluminum powders suggests that theinteraction of the particles in TGA experiments resulting in ameasurable sample overheating causing an accelerated oxidation isunlikely. Consistent TGA observations have been reported for differentpowder samples treated in different instruments and using differentsample holders and sample masses. Furthermore, the same reaction rateswere observed in isothermal experiments (Jones, et al., Journal ofThermal Analysis and Calorimetry 2000; 61(3):805-18). Note also that theoverall oxidation sequence including low-temperature oxidation stepsdistinguished clearly in the TGA experiments was observed for powderswith a broad range of particle sizes, including 10-14 μm sphericalpowders (Trunov, et al. Combustion and Flame 2005; 140(4): 310-8;Trunov, et al. Combustion Theory and Modelling 2006; 10(4):603-23), forwhich self-heating due to oxidation at low temperatures is expected tobe negligible based on their relatively small specific surface area.

The reduced rates of oxidation implied by the SPMS experiments couldhave been affected by the following two issues. The first issue has todo with the sensitivity of the SPMS measurement and its capability toquantify the amount of oxygen present in aluminum particles coated bynatural, 2-3 nm thick oxide layers. Park, et al., J. Physical ChemistryB. 2005; 109(15):7290-9 teach that the method should be sensitive enoughto detect a 1-2 nm oxide coating on aluminum nanoparticles. Despitethat, no oxide was detected for a commercial aluminum nanopowder whilesuch powders always contain 2-3 nm natural oxide layers. A cleardemonstration of the sensitivity of the SPMS technique is thereforeneeded that would indeed show the presence of the 2-3 nm oxide coatingpresent on commercial nanoparticles, for which the presence of the oxidecoating has been established by TEM or another appropriate technique.The second issue is that in the experiments of Park, et al., J. PhysicalChemistry B. 2005; 109(15):7290-9; and Rai, et al., J. Phys. Chem. B2004; 108:14793-5, the particles were subjected to a temperature rampbefore entering the furnace section heated to the target temperature.Because the entire particle feed system was filled with an oxidizinggas, the heating ramp could substantially passivate (oxidize) theparticles resulting in the formation of a thicker and thus moreprotective oxide layer than exists on the starting particles. Based onthe reported experimental conditions, it is estimated that the particlesapproaching the test section at the pre-set temperature, were subjectedto the elevated temperatures for a time period close to 1 s. Thus, theformation of a protective oxide film was inevitable for thenanoparticles produced in situ before these particles entered the testsection pre-heated to the target temperature. For the commercialnanoparticles with the initial protective oxide layer, the oxidation inthe heating ramp section of the apparatus would be comparable to thatoccurring in the calibrated test section. This issue is critical for theparticles generated in situ by arc or laser ablation, for which thepassivation of the bare aluminum surface during the heating ramp canoccur very rapidly. Thus, it is expected that both commercial andin-situ prepared nanoparticles could have been similarly partiallypassivated prior to entering the furnace section. Therefore, themeasured reaction rates in both cases characterize aluminumnanoparticles with initial passivating oxide coating.

Aluminum oxidation has been studied using a differential mobilityanalyzer to measure the density of aluminum particles subjected todifferent temperatures in the oxidizing environment (Rai, et al.,Combustion Theory and Modelling 2006; 10: 843-59). The oxidizedparticles were also examined using a TEM. It was found that particlesexposed to a temperature exceeding 1000° C. become hollow. Based onthose observations, it was proposed that at low temperatures, the rateof oxidation is controlled by inward diffusion of oxygen; above thealuminum melting point, the rate is controlled by aluminum outwarddiffusion. Thus, the oxidation was proposed to occur in two distinctregimes, responsible for the slow and fast oxidation occurring beforeand after aluminum melting, respectively. It was suggested that theconventional thermal analysis techniques are characterizing slowprocesses occurring in the time scale of minutes while the SPMSmeasurements presented by Park, et al., J. Physical Chemistry B. 2005;109(15):7290-9 and Rai, et al., J. Phys. Chem. B 2004; 108:14793-5probed a much faster process completed within 1 s (Rai, et al.,Combustion Theory and Modelling 2006; 10: 843-59). The difference intime scales involved with thermal analysis and SPMS measurements iscertainly real and substantial. In fact, the time scales characteristicof the most practical applications of related energetic nanomaterialsare much shorter than those used in both techniques. Generally, thelonger the time scales involved with a specific experimental technique,the easier it is to overlook a short-duration process. At the same time,such short duration processes can only be meaningfully detected anddescribed by studies of the same material systems at the systematicallyvarying time scales.

An increase in the aluminum oxidation rate at higher temperatures (Rai,et al., Combustion Theory and Modelling 2006; 10: 843-59) is certainlyobserved in all experiments and is in agreement with the concept of theoxidation reaction rate controlled by mass transfer processes. However,the distinct transition from one oxidation regime to another uponaluminum melting is not detected in many TGA measurements that show noincrease in the oxidation rate upon aluminum melting. A short-livedchange in the oxidation mechanism detectable by SPMS but not by TGAcould be associated with a rapid change in the oxidation rate (e.g.,caused by discontinuities or cracks in the passivating oxide layer)followed by a particle self-heating and, therefore, sustained higheroxidation rates. In TGA experiment, the self-heating of the sample iseffectively suppressed that would minimize the effect of a shortduration process.

A systematic study of aluminum oxidation using TGA and analysis of thesamples pre-oxidized to and recovered from specific temperatures waspresented by Trunov, et al. Combustion and Flame 2005; 140(4): 310-8;Trunov, et al. Combustion Theory and Modelling 2006; 10(4):603-23;Trunov, et al. Journal of Physical Chemistry B, 2006; 110(26): 13094-9).As noted above, these TGA results are in agreement with many other TGAmeasurements reported elsewhere. From XRD analysis of the samplesrecovered from specific oxidation temperature conditions, it wasestablished that different Al₂O₃ polymorphs are observed duringdifferent oxidation stages. A diagram in FIG. 17 shows a characteristicthermo-gravimetric analysis (TGA) curve of oxidizing aluminum powder(with micron-sized particles) and the sequence of changes in the aluminascale growing on the particle surface. The entire oxidation process isdivided into four stages and specific processes occurring during eachstage are illustrated schematically. The natural amorphous alumina layercovering the particle initially grows slowly during the low-temperatureoxidation—Stage I. The energy of the oxide-metal interface stabilizesthe amorphous oxide at low temperatures and only up to a criticalthickness of about 5 nm (Levinsky, ASM Intl, 1998; Jeurgens, et al.,Thin Solid Films 2002; 418:89-101). When the critical thickness isapproached or when the temperature becomes sufficiently high, theamorphous oxide transforms into γ-alumina. The density of γ-aluminaexceeds that of amorphous alumina (Levin, et al., J. Am. Ceram. Soc.1998; 81(8):1995-2012), and the smallest observed γ-alumina crystalliteshave a size of about 5 nm (Dwivedi, et al., Journal of Materials ScienceLetters 1985; 4:331-43). Thus, if prior to the phase change thethickness of the amorphous layer was less than 5 nm, the newly formedγ-Al₂O₃ crystallites no longer form a continuous coverage for thealuminum surface. As a result, the rate of oxidation increases rapidlyat the beginning of stage II as shown in FIG. 17. For nanoparticles,this can result in the complete or nearly complete oxidation (see FIG.16). For larger particles, the openings in the oxide coating heal whileonly a small fraction of the metal is oxidized. Upon healing, the rateof oxidation decreases.

Based on the TGA trace shown in FIG. 17, no detectable mass increaseoccurs upon aluminum melting. Eventually, a regular polycrystallinelayer of γ-Al₂O₃ forms by the end of stage II. The growth of γ-Al₂O₃continues in stage III for which the oxidation rate was reported to belimited by the inward grain boundary diffusion of oxygen anions(Jeurgens, et al., J. Appl. Phys. 2002; 92:1649-56; Riano, et al., ActaMaterialia 51: 3617-3634). Growth of the γ-Al₂O₃ layer can beaccompanied by phase transformations into other transition polymorphs,such as δ-Al₂O₃ and θ-Al₂O₃, which have densities very close to that ofγ-Al₂O₃ (Levin, et al., J. Am. Ceram. Soc. 1998; 81(8):1995-2012). Suchtransitions are not expected to affect the oxidation rate significantly.Other processes are largely irrelevant for powders or materials withaluminum nanodomains, which are oxidized completely well before thealumina layer becomes unstable. However, for completeness, the processesoccurring at higher temperatures are reviewed briefly. Stage III endswhen the increased temperature destabilizes the transition betweenalumina polymorphs. The particles stabilize at elevated temperatures andstill denser a-alumina polymorph starts forming by the end of stage III.Thus, strictly speaking, stage III can be further broken down into threeseparate sub-stages: growth of γ-Al₂O₃, transformations to δ and θalumina polymorphs, and transformation to a denser α-Al₂O₃ polymorph.Stage IV is considered to start when the oxide scale is completelytransformed to α-alumina. When the initial α-Al₂O₃ crystallites form atthe end of stage III, the thickness of the γ-Al₂O₃ layer decreases, andthe oxidation rate increases momentarily. When most of the oxide layeris transformed to the coarse and denser α-Al₂O₃ crystallites, resultingin continuous polycrystalline coverage, grain boundary diffusionprocesses slow down and the oxidation rate decreases rapidly.

Trunov, et al. Combustion Theory and Modelling 2006; 10(4):603-23)report the kinetic relations for the polymorphic phase changes occurringin alumina films coating aluminum powders determined from TGAexperiments. Thus, a quantitative description of aluminum oxidation ingaseous oxygen for particles and domains of different sizes wasobtained.

Reactions Between Condensed Components

Three main types of related reactive systems include thermite,intermetallic, and metal-metalloid compounds. Among these three classesof composites, reactions in thermites are of most interest for energeticmaterials applications and they have attracted the most attention in theresearch community. Reactions mechanisms in highly exothermicmetal-metalloid systems (e.g., B—Ti or B—Zr nanocomposites) are amongthe least studied. Therefore, the discussion below focuses primarily onthe reactions in thermites.

The sequence of processes occurring in aluminum oxide and affectingaluminum oxidation in gaseous oxygen described above is expected toremain generally valid for aluminum reactions with various oxidizers,including metal oxides in nanocomposite thermites. Clearly, the kineticparameters of individual processes are expected to be affected. Thestability ranges of different alumina polymorphs can also change as aresult of presence of other metals or oxides. Formation of alloys orternary oxides can result in even more complex and multistage reactions.For thermites, reduction of metal oxides most often does not occur inone step; for example, as noted above, it is reasonable to expect thatMoO₃ reduction to Mo occurs through formation of Mo₉O₂₆, Mo₄O₁₁, andMoO₂ according to the Mo—O phase diagram (Levinsky, ASM Intl, 1998).Formation of each individual sub-oxide is expected to alter the reactionkinetics. Current research is primarily aimed at quantitativeexperimental characterization of these more complex reaction mechanisms.

Thermal analysis remains the most important tool in studying reactionsin thermite-type nanocomposites and in nanocomposite materials employingother types of exothermic solid-solid reactions. While thenano-dimensions themselves may not necessarily result in new reactionmechanisms, the exothermic processes that occur very slowly and remainundetected for coarser materials can become dominant ignition triggersfor nanocomposite materials with the identical bulk compositions. Infact, DSC and DTA measurements for many nanocomposite materials detectreaction onsets at low temperatures and will be useful in characterizingthe respective reaction kinetics. Despite many advantages, the thermalanalysis techniques are poorly suited for detecting short-livedprocesses that might result in self-heating for individual particles orsmall samples. Straightforward extrapolation of the kinetic behaviorobserved in thermo-analytical experiments to much shorted time scales isnot meaningful. Instead, reaction mechanisms identified from detailedthermo-analytical studies combined with other experimental techniquesshould be elucidated and used to predict the reactions occurring inpractical applications.

Reactions in aluminum-copper oxide nanofoils, produced by magnetronsputtering, were studied in a series of experiments presented in refs.(Blobaum, et al., Journal of Applied Physics 2003; 94(5):2915-22;Blobaum, et al., Journal of Applied Physics 2003; 94(5):2923-9).Differential thermal analysis was used in addition to TEM and Augerprofiling of the partially reacted samples. The preparation ofmultilayered thermite nanofoils involves elevated temperatures soformation of pure CuO was not possible. It was reported that thesputter-deposited copper oxide had the structure of the mineralparamelaconite, Cu₄O₃. The Al/Cu₄O₃ molar ratio in the foils isapproximately 2.5, so that based on the final products of Al₂O₃ and Cu,the system is somewhat aluminum-rich. Furthermore, aluminum waspartially oxidized and copper oxide was partially reduced during thedeposition so that that Al and Cu₄O₃ bilayers with combined thickness ofabout 1 μm were separated by approximately 100 nm thick interface layersin which the concentrations of components were continuously changing, aswas confirmed by the Auger profiles of Al, Cu, and O (Blobaum, et al.,Journal of Applied Physics 2003; 94(5):2915-22). A narrow region of theinterface was identified as an amorphous or nanocrystalline Al₂O₃.

DTA analysis showed that the exothermic reaction in such nanofoilsoccurred in two steps, with the peaks around 625° C. (890K) and 835° C.(1110 K). Based on the DTA trace presented by Blobaum, et al., Journalof Applied Physics 2003; 94(5):2915-22; Blobaum, et al., Journal ofApplied Physics 2003; 94(5):2923-9), and reproduced here as a dashedline in FIG. 18, the onset of an exothermic reaction can be assigned toabout 500 K. There is a broad exothermic shoulder beginning at about470° C. (about 745 K) in addition to the two large peaks mentionedabove. Based on detailed XRD and Auger studies of the samples quenchedbefore the first exothermic peak, the authors concluded that most of theparamelaconite was transformed into a mixture of CuO and Cu₂O. Based onfurther analyses of the partially reacted foils, likely rate-determiningprocesses for each of the two reaction steps were proposed. In the firstexothermic reaction, the lateral growth of Al₂O₃ nuclei was proposed tocontrol the reaction rate. The reaction ended when a continuous Al₂O₃layer was formed. Note the similarity of the temperature ranges for theend of this reaction step described in (Blobaum, et al., Journal ofApplied Physics 2003; 94(5):2923-9) and the end of aluminum oxidationstep II illustrated in FIG. 17, which was also assigned to the formationof a continuous γ-Al₂O₃ layer (Trunov, et al. Combustion and Flame 2005;140(4): 310-8; Trunov, et al. Combustion Theory and Modelling 2006;10(4):603-23). The second exothermic peak for the Al—CuO nanofoils wasproposed to be controlled by either diffusion of O through the Al₂O₃ orby thickening of the Cu product by a nonuniform reduction in copperoxide serving as the oxygen source. This interpretation is againconsistent with that discussed in refs. (Trunov, et al. Combustion andFlame 2005; 140(4): 310-8; Trunov, et al. Combustion Theory andModelling 2006; 10(4):603-23) for aluminum, where the respectiveoxidation step was proposed to be controlled by the diffusion-controlledgrowth of γ-Al₂O₃.

Reactions in a similar Al—CuO nanocomposite material prepared by ARMwere studied by Umbrajkar, et al., Thermochimica Acta 2006; 451:34-43.The bulk composition was stoichiometric, i.e., 2Al+3CuO and detailed DSCstudies accompanied by XRD analysis of the samples oxidized to differenttemperatures were reported. A DSC trace from Umbrajkar, et al.,Thermochimica Acta 2006; 451:34-43 is directly comparable to thatpresented in (Blobaum, et al., Journal of Applied Physics 2003;94(5):2915-22; Blobaum, et al., Journal of Applied Physics 2003;94(5):2923-9)—shown in FIG. 18. The onset of exothermic reactions forthe ARM-prepared material occurs at a noticeably lower temperature andthe low-temperature shoulder is resolved to include at least two broadpeaks. Only one of the two higher temperature peaks observed fornanofoils (Blobaum, et al., Journal of Applied Physics 2003;94(5):2915-22; Blobaum, et al., Journal of Applied Physics 2003;94(5):2923-9) is reported for the nanocomposite powders (Umbrajkar, etal., Thermochimica Acta 2006; 451:34-43), for which the temperature scanwas stopped at 1013 K (740° C.). In addition, two weak endothermic peaksare observed between 800 and 900 K, corresponding to the eutectics withCuAl₂ and Cu₉Al₄, respectively. The presence of intermetallic phases inthe reaction products was confirmed by XRD. Both the better resolvedlow-temperature shoulder and the formation of intermetallic phases inthe Al—Cu system (requiring the presence of metallic Cu), suggest asubstantially greater reaction rate at low temperatures for materialsprepared by ARM as compared to the nanofoils. Because of the higherrate, a greater fraction of the copper oxide is reduced at the sametemperatures for the ARM-prepared materials. The metallic copper formedis then capable of reacting with aluminum and producing the observedintermetallic phases. This enhancement of reactivity for theARM-prepared materials compared to respective nanofoils can be explainedby two main factors. First, the magnetron sputtered copper oxide waspartially reacting with aluminum during its deposition and the initialphase of the deposited copper oxide was partially reduced. Second, thespatial scale of mixing between the metal and the copper oxide in thenanofoils was likely coarser than in the three-dimensional nanocompositeparticles produced by ARM.

Studies of the partially reacted samples recovered from differenttemperatures presented by Umbrajkar, et al., Thermochimica Acta 2006;451:34-43 were inadequate to propose a specific reaction mechanismcorresponding to the complex DSC pattern. It was clear that thereduction of CuO occurred through formation of Cu₂O followed by theformation of Al—Cu alloys and metallic copper. Produced aluminum oxidephases were poorly crystalline and only weak peaks of γ-Al₂O₃ weredetected in the XRD patterns of the partially reacted samples. Thus,instead of proposing a specific reaction mechanism, the reaction wassimply presented as a superposition of at least four overlappingindividual reactions. For each individual contribution, the genericreaction type was selected and activation energy was estimated specifiedby matching experimental DSC results collected at different heatingrates.

Similarly to reactions for Al—CuO thermites, reactions for Al—MoO₃thermites were studied using thermal analysis for nanocompositematerials prepared by different techniques. DSC curves forstoichiometric 2Al+MoO₃ prepared by ARM (Schoenitz, et al. Journal ofPropulsion and Power 2007; 23(4):683-7) and for a mixture prepared byultrasonication of nanosized Al and MoO₃ powders at the equivalenceratio of 1.2 (Sun, et al., Thermochimica Acta 2006; 444(2): 117-27) areshown in FIG. 19. The equivalence ratio for the ultrasonicatednanopowders was selected based on earlier experiments (Granier, et al.,Combustion and Flame 2004; 138(4):373-83) showing the maximum reactivityof the respective nano-thermite prepared at that equivalence ratio.

Comparison of the DSC traces shown in FIG. 19 to each other shows thatthe exothermic reaction starts at lower temperatures and occurs moreactively for the materials prepared by ARM. In both cases, theexothermic features are broad and individual peaks are difficult toseparate. A closer inspection of the traces shown in FIG. 19 indicatesthat the DSC trace for the ARM-produced material could be constructed byoverlapping the trace observed for the mixed nanopowders, with anadditional, very broad exothermic feature starting at about 500 K andmonotonously increasing up to about 900 K. Without specifying thereaction mechanism, the DSC traces for ARM-prepared materials wereproposed to be modeled as a superposition of four overlapping reactions.Three of these reactions described individual smaller peaks, similar tothose detectable from the DSC trace for mixed nanopowders. Each of thesethree peaks was described as a first order reaction, with the activationenergies of 209, 211, and 373 kJ/mol, respectively (Schoenitz, et al.Journal of Propulsion and Power 2007; 23(4):683-7). The fourth, broadfeature, not detectable for the mixed nanopowder samples, wasapproximated by a low activation energy (90 kJ/mol) Jander type reactioncommonly describing 3-dimensional diffusion. Note that the activationenergy for the strongest peak observed was about 240 kJ/mol. Based onthe DSC traces recorded at the same heating rate and shown in FIG. 19,for the ARM-prepared materials this peak is located between the thirdand fourth identified peaks, for which respective activation energieswere 211 and 373 kJ/mol, respectively (Schoenitz, et al. Journal ofPropulsion and Power 2007; 23(4):683-7).

The mechanisms of these and other exothermic reactions in nanocompositereactive materials are complex and involve a number of overlappingexothermic processes. Two fundamentally different approaches arepossible to describe such reactions. One approach is based onunderstanding the individual processes occurring in the reactingmaterials. Such understanding is only possible based on complexinvestigations involving various material probing techniques combinedwith thermal analysis. The only example of this approach is reported byBlobaum, et al., Journal of Applied Physics 2003; 94(5):2915-22;Blobaum, et al., Journal of Applied Physics 2003; 94(5):2923-9) forthermite nanofoils.

Reactions in intermetallic systems have been studied for many years andgood reviews of the current approaches and understanding can be found inthe literature, e.g., (Mehrer, JIM 1996; 37(6): 1259-80; Laurila, etal., Materials Science and Engineering R: Reports 2005; 49(1-2):1-60;Morsi, Materials Science and Engineering A 2001; 299(1-2):1-15). Theresearch is focused on understanding of the combined processes ofdiffusion, formation of solid solutions, and the formation of theintermetallic compounds. One of the most reactive intermetallic systemsis Al—Ni and despite extensive previous studies (Morsi, MaterialsScience and Engineering A 2001; 299(1-2):1-15), the reaction mechanismsin this system remain the subject of debate. This reaction was recentlystudied for nanofoil materials (Blobaum, et al., Acta Materialia 2003;51:3871-84). The mechanism of the Al—Ni reaction is complex and includesformation of multiple intermediate phases, so that detailed studiescombining thermal analysis with recovery and characterization of samplesheated to intermediate temperatures, are extremely useful. The simplemorphology of the nanofoils combined with a large reactive interfacearea enabled researchers to observe and study reaction steps that aredifficult to characterize for other types of composite materials.Different phases (e.g., Al₉Ni₂, Al₃Ni) were observed to form as thefirst intermediate reaction product in the bilayers of differentthicknesses, which may reflect the effect of nanoscale dimensions on thethermodynamic stabilities of different compounds—mentioned above.

Ignition Studies

The exothermic heterogeneous reactions are responsible for ignition ofreactive nanocomposite materials under practically useful conditions.The salient feature of ignition is that the materials are heated rapidlywith powerful heat sources, such as a detonation front produced by anignition primer or as an expanding fire ball generated by a highexplosive. Respective heating rates are of the order of 10⁶-10⁷ K/s.Thus, experimental validations of the ignition models must generatecomparable heating rates while enabling controllable and reproducibleconditions and accurate ignition detection.

In addition to reproducing the high heating rate, the entirenanocomposite should be heated and ignited in the ignition experiment,as opposed to selective heating (and possible volatilization ordecomposition) of one component, which could result in differentreaction kinetics. The uniform heating of composite material is readilyachieved in thermo-analytical experiments with very low heating rates,but may present a challenge when the heating rates increase. Finally,for materials prepared as mixed nanopowders, the sample porosity canvary widely between experiments which could also substantially changethe ignition behavior.

Lasers offer the capability of readily adjustable and well-controlledenergy transfer to reactive materials with the range of power necessaryto achieve the heating rates of interest. Lasers are now commonly usedfor laboratory studies of material ignition, e.g., (Dimitriou, et al.,AIChE Journal 1989; 35(7): 1085-96; Zakharov, et al., Izvestiya AkademiiNauk SSSR, Seriya Fizicheskaya 1991; 55(6): 1198-201; Fetherolf, et al.,Proceedings of the 16^(th) International Pyrotechnics Seminar, 1991, p.675-90; Kuo, et al., Combustion and Flame 1993, 95(4):351-61; Ahmad, etal., Propellants, Explosives, Pyrotechnics 2001; 26(5):235-45; Ali, etal., Combustion Science and Technology 2003; 175(8):1551-71). Mostsignificant issues for the nanocomposite materials are the consistencyin the sample preparation and differences in efficiency of absorption ofthe laser energy by different material components (e.g., metals versusoxides or particles of different sizes present in the material). Theefficiency of absorption of the near-infra red and visible spectra bynanoparticles of Al and B embedded in nitrocellulose or Teflon oxidizerswas studied by Yang, et al., Propellants, Explosives, Pyrotechnics 2005;30(3):171-7. It was found that absorption strength is greater for Alnanoparticles as compared to bulk Al. A different experimental approachwas used to assess the scattering and absorption efficiency of n-Al andnanosized MoO₃ powders (Begley, et al., Journal of Heat Transfer 2007;129(5):624-33). It was found that close-packed nanopowder of MoO₃scatters most of the incident light while about ⅔ of the incident lightare absorbed in a similarly packed n-Al. The experimental approach usedin (Begley, et al., Journal of Heat Transfer 2007; 129(5):624-33)required preparation of a 1-D slab with a moderate optical thickness,which proved to be difficult for n-Al.

In some laser ignition experiments, nanosecond or even shorter laserpulses are used resulting in significantly higher heating rates oftenexceeding 10⁹ K/s (Yang, et al., Journal of Physical Chemistry B 2003;107(19):4485-93; Wang, et al., Propellants, Explosives, Pyrotechnics2005; 30(2):148-55; Zamkov, et al., Journal of Physical Chemistry C2007; 111(28):10278-84; Mileham, et al., Journal of Physical Chemistry C2007; 111(45):16883-8 Yang, et al., Journal of Physical Chemistry B2003; 107(19):4485-93; Wang, et al., Propellants, Explosives,Pyrotechnics 2005; 30(2): 148-55; Zamkov, et al., Journal of PhysicalChemistry C 2007; 111(28): 10278-84; Mileham, et al., Journal ofPhysical Chemistry C 2007; 111(45): 16883-8).

An experimental study on laser ignition of Al—MoO₃ nanocompositethermites prepared by ultrasonic mixing of respective nanopowders wasreported by Granier, et al., Combustion and Flame 2004; 138(4):373-83. A50 W CO₂ laser was used to ignite pellets pressed to 38% TMD preparedfrom different size aluminum powders and different equivalence ratios.Ignition delay was measured as the time between the laser onset and thefirst detection of optical emission by the pellet. The results ofexperiments with nearly stoichiometric thermites prepared using aluminumparticles with different sizes are illustrated in FIG. 20. The ignitiondelays were unaffected by the particle size for nanopowders. Compared tothe 100 nm powder, the ignition delays appeared to increase roughly byan order of magnitude with an order of magnitude increase in thealuminum particle sizes. The effect of equivalence ratio was studiedusing three different aluminum nanopowders (nominal sizes of 30, 40, and108 nm). The results did not show substantial differences in theignition delays which were around several tens of milliseconds for therange of equivalence ratios of 0.5-2. Ignition delays increasednoticeably when the samples were Al rich (equivalence ratios greaterthan 2) and when micron-sized Al powders replaced the n-Al.

These results do not seem to provide the data conducive for directvalidation of any specific ignition model. The difficulties in the datainterpretation presented by Granier, et al., Combustion and Flame 2004;138(4):373-83 are instructive. The millisecond ignition delays could bedescribed theoretically by modeling the heat transfer processes of thepellet heated by the laser; however, such a description requires thevalue of the bulk thermal conductivity of the pellet, which is verydifficult to either predict or measure. This difficulty is common forall reactive nanocomposite materials. The absence of a clear effect ofthe size of aluminum nanoparticles on the ignition delay most likelyresults from the differences in the mixing quality of the thermitecompositions prepared with different nanopowders. It is, for example,reasonable to expect that the finer nanoparticles were mixed lessuniformly with MoO₃ than coarser ones, so that the effect of reducedparticle size was effectively negated. Mixing uniformity is determinedby the sample preparation method, and focused efforts will be necessaryto evaluate it quantitatively. The specific details of CO₂ laserradiation interaction with either nano-aluminum or nano-MoO₃ are alsopoorly known and it is unclear what the differences might be without aspecific study. Finally, in addition to optical measurements of theigniting sample emission, temperature measurement of the sample wasattempted by Granier, et al., Combustion and Flame 2004; 138(4):373-83using thermocouples. The reported temperature traces suggested ignitionof thermites prepared with nanopowders at around 100° C. while the samethermites prepared with the micron-sized powders were reported to igniteat 610° C. As reported earlier in the same article, ignition for bothtypes of samples was detected optically using a high speed video camera.Without a very sensitive IR detector, however, optical emission isextremely hard to detect at 100° C. and even at 610° C. These lowtemperatures are also inconsistent with any other measurements onignition of materials containing Al or nano-Al as fuels. The authors of(Granier, et al., Combustion and Flame 2004; 138(4):373-83) admit thatthe temperature curves presented are inaccurate and do not account fortransient effects and other experimental errors but suggest that suchdata can be used for qualitative comparison between different sizealuminum powders.

An attempt to obtain a more quantitative characterization of ignitionkinetics for nanocomposite reactive materials using laser ignition wasmade by Hunt, et al., Journal of Applied Physics 2005;98:34909-1-034909-8. The nanocomposite system considered was Al—Ni andthe material was prepared by ultrasonic mixing of the respectivenanopowders. Cylindrical pellets with 55-60% TMD were then pressed andignited using a CO₂ laser beam. The variation of the measured ignitiontemperature as a function of the heating rate was obtained. Activationenergies were recovered using isoconversion data processing. A dramaticdifference in the ignition activation energy for micron-sized andnanosized Al powders was reported. Thermocouple measurements similar tothose of Granier, et al., Combustion and Flame 2004; 138(4):373-83 wereused to identify the ignition temperature. Even though the heating rateswere rather low, on the order of 1-10 K/min, the error in thethermocouple measurement still needs to be quantified in order for thereported ignition kinetics to be meaningful. Unfortunately, no erroranalysis for the thermocouple measurements was provided. A summary ofactivation energies relevant for the Al—Ni reactions, including thoseoccurring in the nanofoils, is presented by Blobaum, et al., ActaMaterialia 2003; 51:3871-84, which does not include the unusually lowactivation energies for nanocomposites reported by Hunt, et al., Journalof Applied Physics 2005; 98:34909-1-034909-8. Generally, a decrease inthe particle size is not expected to reduce dramatically the activationenergy for any heterogeneous reaction; instead, it is expected tosubstantially increase the value of the pre-exponent factor in theArrhenius-type reaction kinetic description. Surprisingly, the values ofpre-exponent factors, also determined in (Hunt, et al., Journal ofApplied Physics 2005; 98:34909-1-034909-8), are of the same order ofmagnitude for all micron-sized and nano-sized powders used to preparecomposite samples. Finally, the heating rates achieved in (Hunt, et al.,Journal of Applied Physics 2005; 98:34909-1-034909-8) are typical forthermal analysis experiments and much higher rates are generally desiredin laser ignition configurations.

Heterogeneous shock tube experiments offer another configuration inwhich well-defined and high heating rates can be achieved (Bazyn, etal., Combustion Science and Technology 2007; 179:457-76). In suchexperiments, a powder-like sample is placed or injected near the wallthe “driven” section of a shock tube and the incident and reflectedshock waves pass over the sample quickly and heat it in two sequentialsteps, closely following each other. This way, the sample is nearlyinstantaneously introduced into a hot gas and its temperature historycan be relatively easily calculated with a convective heating model. Thesample ignition and combustion are monitored optically and the ignitiondelay is quantified considering the well-known timing of the reflectedshock. This technique is not suitable for the nanocomposite samplesprepared as mixed, low density nanopowders because such materialsdisintegrate after being dispersed by the shock wave. On the other hand,this technique is well suited to study ignition kinetics for nano-sizedaluminum powders as well for the fully dense nanocomposite powdersprepared by ARM. The results of measurements presented by Bazyn, et al.,Combustion Science and Technology 2007; 179:457-76 are shown in FIG. 21.Ignition delays measured by Bazyn, et al., Combustion Science andTechnology 2007; 179:457-76 for nanosized aluminum powders andnanocomposite thermites were of the order of a microsecond, whilemillisecond delays were observed for a micron-sized aluminum powder (cf.inset in FIG. 21.) The main combustion peaks in the powder emission werealso shifted in time. The most rapid combustion was observed for theAl—MoO₃ nanocomposite thermite, followed by nearly coinciding peaks forn-Al (ALEX) and Al—Fe₂O₃ nanocomposite thermite. The combustion peak forthe micron-sized Al powder was observed after the longest delay.

The measurements of ignition kinetics are strongly affected by theparticle size distribution in the sample tested and by possible particleagglomeration (especially significant for nanosized powders). Thus, inorder to directly validate a quantitative ignition model, detailedcharacterization of the particle size distribution is necessary and theparticle size distribution effect needs to be explicitly addressed inboth experimental validations and respective calculations.

Experimental studies of the ignition of powder-like samples coated on anelectrically heated filament were described by Umbrajkar, et al.,Propellants, Explosives, Pyrotechnics 2007; 32(1):32-41; Umbrajkar, etal., Propellants Explosives and Pyrotechnics 2006: 31(5):382-9;Umbrajkar, et al., Thermochimica Acta 2006; 451:34-43; Ward, et al.,International Journal of Heat and Mass Transfer, 2006;49(25-26):4943-54; and Shoshin, et al., Combustion and Flame 2006;144(4):688-97. The filament temperature was measured in real time usingan infrared pyrometer focused on an uncoated filament surface adjacentto the powder coating. The ignition instant was detected optically usinga second photo sensor focused on the powder coating. The method provideswell-controlled heating rates between 10² and 10⁵ K/s. A detailedanalysis of the heat transfer between the filament and the thin layer ofthe powder coating was presented in (Ward, et al., International Journalof Heat and Mass Transfer, 2006; 49(25-26):4943-54) and ignitionkinetics for various nanocomposite samples was reported in (Umbrajkar,et al., Propellants, Explosives, Pyrotechnics 2007; 32(1):32-41;Umbrajkar, et al., Propellants Explosives and Pyrotechnics 2006:31(5):382-9; Umbrajkar, et al., Thermochimica Acta 2006; 451:34-43).Ignition temperatures for nanocomposite materials were found to vary ina general range of 600-1000 K (Umbrajkar, et al., Thermochimica Acta2006; 451:34-43; Schoenitz, et al. Journal of Propulsion and Power 2007;23(4):683-7).

The kinetics of material ignition determined from experiments performedwith different heating rates were compared with the kinetics of variousexothermic events observed with the same materials from a thermalanalysis (DSC), performed over a different range of heating rates. Anexample of such comparison for Al—MoO₃ thermites is shown in FIG. 22.The data shown in the coordinates corresponding to the isoconversionprocessing: the logarithm of the ratio of the temperature square overthe heating rate (T²/b) is plotted against the inverse temperature(1/T). For clarity, a top axis is added showing the actual temperaturescorresponding to the horizontal coordinate. The DSC curve processed issimilar to the top curve shown in FIG. 19 and includes a number ofexothermic features. Four of the most prominent features were processedfrom multiple heating rate DSC experiments to obtain a family of linescorresponding to the low heating rate range in FIG. 22. The heatedfilament ignition experiments, performed at much higher heating ratesare shown in the same plot. One immediate conclusion from comparing thefilament ignition and DSC inferred kinetic trends is that the strongestpeak (represented by open circles and labeled “peak 4” in FIG. 22)observed in the DSC signal at higher temperatures does not really affectignition of these powders, which occurs at lower temperatures despitemuch higher heating rates. Thus, the processes occurring at lowertemperatures must be responsible for triggering the material ignition.Weaker exothermic events are indeed observed in the DSC curves at lowertemperatures (cf. FIG. 19). None of the trends obtained from DSC for thelow-temperature events can be directly correlated with the kinetic trendobserved for the filament ignition experiments, however. The most likelyexplanation is that the low-temperature exothermic features observed inthe DSC experiments do not represent individual reaction steps butrather are compounded events. The direct extrapolation of kinetic trendsfor such compounded events observed in the narrow range of heating ratesavailable for DSC measurements to the much higher heating rates (as canbe implied by FIG. 22) is poorly justified. Thus, the low-temperatureexothermic events need to better characterized in order to predictignition of such materials. An initial analysis and modeling approachwere provided by Umbrajkar, et al., Thermochimica Acta 2006; 451:34-43and Schoenitz, et al. Journal of Propulsion and Power 2007; 23(4):683-7based on a model of multiple overlapping processes that occur during thesample heating. Isoconversion processing may not be very useful foranalysis of such reactions involving steps that overlap andsubstantially affect one another (Umbrajkar, et al., Thermocimica Acta,Submitted, 2008).

Combustion Studies Laboratory Tests

Laboratory combustion experiments may establish a clear and reproducibledifference between combustion of such materials and that of referencematerials using the same elemental or molecular compounds which were notmixed on the nanoscale. A number of semi-qualitative measures ofreactivity, primarily involving one or another variant of an open trayburn experiment, e.g., (Pantoya, et al., Propellants, Explosives,Pyrotechnics 2005; 30 (1):53-62; Clapsaddle, et al., Materials ResearchSociety Symposium Proceedings 2003; 800:91-6; Schoenitz, et al.,Proceedings of the Combustion Institute 2005; 30:2071-8; Prentice, etal., J. Phys. Chem. B 2005; 109:20180-5; Kwon, et al., Combustion andFlame 2003; 133:385-91; Moore, et al., Propellants, Explosives,Pyrotechnics 2004; 29(2): 106-11; Perry, et al., Propellants,Explosives, Pyrotechnics 2004; 29(2):99-105) were performed. A sample ofmaterial, typically a powder, was placed in an arbitrarily selected opensample holder, often an elongated channel, and ignited using a pilotflame, hot wire, piezoelectric igniter, laser, or other appropriatedevice. The ensuing flame was typically visualized using high speedvideo. A characteristic photograph of such an experiment (Tillotson, etal., Journal of Non-Crystalline Solids 2001; 285(1-3):338-45). Sampleswith reactive components of different sizes, mixed on different scales,mixed using different techniques, or prepared using different synthesisapproaches, were examined. Optical temperature measurements were alsoattempted (Kwon, et al., Combustion and Flame 2003; 133:385-91; Moore,et al., Propellants, Explosives, Pyrotechnics 2004; 29(2):106-11).Generally, qualitative differences in combustion were observed—with theapparent flame speeds for nanocomposite materials several orders ofmagnitude greater than the speeds measured for similar materials mixedon the micron or coarser scales. From these studies, flame speeds forthe nanocomposite materials, with the same or similar compositions, varyover a broad range from ˜0.1-10³ m/s and appear to be most affected bythe packing density of the sample. Unlike conventional explosives, forreactive nanomaterials, a higher sample density always results in alower visible flame speed. The visible flame speed was observed to bestrongly affected by the size distributions and/or types of fuel(Pantoya, et al., Propellants, Explosives, Pyrotechnics 2005; 30(1):53-62; Moore, et al., Propellants, Explosives, Pyrotechnics 2004;29(2):106-11; Puszynski, Materials Research SocietySymposium—Proceedings 2003; 800:223-32) and oxidizer (Plantier, et al.,Combustion and Flame 2005; 140:299-309; Prentice, et al., J. Phys. Chem.B 2005; 109:20180-5) particles, method and uniformity of mixingnanosized fuel and oxidizer powders (Puszynski, et al., MaterialsResearch Society Symposium Proceedings 2006; 896:147-58, Prentice, etal., J. Phys. Chem. B 2005; 109:20180-5), and scale of mixing achievedin the fully dense nanocomposites prepared by ARM (Schoenitz, et al.,Proceedings of the Combustion Institute 2005; 30:2071-8).

As the initial qualitative features of combustion of nanocompositematerials became established, open tray experiments became moreinstrumented to obtain more quantitative and meaningful combustioncharacteristics. Photodiodes with collimated inputs were used toregister flame arrival instant and obtain a more accurate measurement ofthe flame speed (Puszynski, Proceedings of the 29^(th) InternationalPyrotechnics Seminar. Publisher: Defense Science & TechnologyOrganization, Pyrotechnics Group; 2002, p. 191-202; Moore, et al.,Journal of Propulsion and Power 2007; 23(1):181-5; Puszynski, MaterialsResearch Society Symposium—Proceedings 2003; 800:223-32). Pressuresensors were also used to obtain the pressure signatures of thepropagating flames (Sanders, et al., Journal of Propulsion and Power2007; 23(4):707-14). Poor reproducibility between the flame speedmeasurements in open tray experiments was often observed and it wassuggested that the setup could be improved by installing a periodicsequence of baffles with small openings along the channel in which thesample is placed and ignited (Puszynski, Proceedings of the 29^(th)International Pyrotechnics Seminar. Publisher: Defense Science &Technology Organization, Pyrotechnics Group; 2002, p. 191-202; Walter,et al., Journal of Propulsion and Power 2007; 23(4):645-50). The baffleswere meant to minimize the effect of removal or suspension of unignitedfree powder as a result of the strong convective flows produced by thepropagating flames. Improved reproducibility was indeed achieved;however, the measured flame velocities decreased substantially and themechanism of flame propagation remained unclear.

Further modifications of the experimental methodology have includedusing cylindrical tubes in which the sample was packed and ignited(Bockmon, et al., Journal of Applied Physics 2005; 98(6): 1-7 art. no.064903; Sanders, et al., Journal of Propulsion and Power 2007;23(4):707-14). Such experiments are often referred to as a “confinedsample burn.” Usually the tubes are equipped with multiple side openingsfor pressure transducers and fiber optics cables feeding the flameemission signal to photodiodes for kinetics and emission analyses. Asthe ends of the tubes are typically open, such confinement of thecombustion materials is only partial. Recently, flame propagation inmicrochannels was reported for nanocomposite thermite (Son, et al.,Journal of Propulsion and Power 2007; 23(4):715-21) andnano-aluminum/water systems (Risha, et al., Proceedings of theCombustion Institute 2007; 31 II:2029-36). A sequence of the high speedimages illustrating combustion of an Al—MoO₃ thermite prepared by mixingrespective nanopowders in a narrow, 2 mm tube (Son, et al., Journal ofPropulsion and Power 2007; 23(4):715-21). In a related study, electricalconductivity was measured for a burning a nanocomposite sample partiallyconfined between two brass electrodes (Tasker, et al., Journal ofApplied Physics 2006; 99:023705). Electrical contacts were used tomeasure the flame propagation speed in nanocomposite aluminum-Teflon®materials prepared by mechanical milling (Dolgoborodov, et al.,Khimicheskaya Fizika 2004; 23(9):85-9; Dolgoborodov, et al., JETPLetters 2005; 81(7):311-4; Dolgoborodov, et al., Khimicheskaya Fizika2007; 26(12):40-5) and placed in channels at relatively low densities(typically under 50% TMD). The measured speeds of flame propagationvaried in the range of 700-1280 m/s, while independent measurementsestablished that the speed of sound in a similarly prepared sample isonly about 100 m/s. Thus, the feasibility of detonation in a nano-scaleAl-Teflon mixture was reported. As in the other similar measurements,the measured flame propagation speed was observed to decrease with theincrease in mixture density (Dolgoborodov, et al., Khimicheskaya Fizika2007; 26(12):40-5).

Despite a relatively large number of reports, observations based on theapparent flame speed for nanocomposite materials are rather limited. Itwas reported that the dilution of the nanocomposite materials with inertadditives (Prentice, et al., J. Phys. Chem. B 2005; 109:20180-5) resultsin an appreciable reduction of the flame speed. It was also observedthat preparing off-stoichiometric compounds can result in an increasedflame speed for slightly metal rich cases, e.g., for Al—CuO thermite(Walter, et al., Journal of Propulsion and Power 2007; 23(4):645-50) orfor Al—MoO₃ thermite (Granier, et al., Combustion and Flame 2004;138(4):373-83; Bockmon, et al., Journal of Applied Physics 2005; 98(6):1-7 art. no. 064903). On the other hand, it has been reported that anincreased flame speed can be maintained when a nanosized aluminum powderis blended with a micron-sized powder. The flame speed for ananocomposite with the fuel prepared as a blend of micron and nano-sizedpowders matched that of the nanocomposite prepared with pure aluminumnanopowder when about 60% of nanopowder was added (Moore, et al.,Journal of Propulsion and Power 2007; 23(1):181-5). As noted above, itwas observed that the sample packing density affects the apparent flamespeed dramatically; for example a difference by two orders of magnitudewas observed for loose and “packed” samples, with respective densitiesof 5-10% and 35-55% TMD (Sanders, et al., Journal of Propulsion andPower 2007; 23(4):707-14). It was also reported in (Sanders, et al.,Journal of Propulsion and Power 2007; 23(4):707-14) that the effect ofpacking density on the pressure produced by the burning nanocompositethermites was opposite to the effect of packing density on the apparentflame propagation velocity.

Interpretations of the apparent flame propagation measurements, oftenshowing supersonic values of the flame speeds for the nanocompositematerials, are difficult. The convective flow patterns are betterdefined for the partially confined samples, but the thermal propertiesof the loosely packed or even pressed nanocomposite samples critical fordescription of the flame propagation remain poorly quantified. Theseproperties are likely changing dramatically as the propagating flameheats up the sample and/or as the sample is compressed by the pressurewave generated by the flame front. The magnitude of such changes islikely comparable to that observed for the electrical properties of thenanocomposite mixtures affected by the flame propagation (Tasker, etal., Journal of Applied Physics 2006; 99:023705). Simplified heattransfer models for flame propagation in confined and unconfinednanocomposite materials are offered in refs. (Walter, et al., Journal ofPropulsion and Power 2007; 23(4):645-50; Wilson, et al., AIAA Paper2003-4536, 2003; Wilson, et al., AIAA Paper 2005-0275, 2005). Thesemodels operate with lumped time scales and bulk burn rates and do notinclude the inherent reaction kinetics and varied transport propertiesof the reacting materials. Such models are attractive for practicalusers but the multiple simplifying assumptions made are not currentlyjustified. It is possible to select the adjustable parameters to fit aselected experimental data set, it is unreasonable to expect that thesame parameters will be useful in predicting combustion behavior formaterials with altered properties or burning in a differentconfiguration.

Changes in the thermodynamic and transport properties of the reactingmaterials need to be considered. For example, an enhanced heat transferis expected when gasified reduced metal formed in a thermite reaction,such as Mo (or Cu and Bi) condenses on unignited or burning particles(Son, et al., Journal of Propulsion and Power 2007; 23(4):715-21). Inanother recent report, it was suggested (Sanders, et al., Journal ofPropulsion and Power 2007; 23(4):707-14) that the flame propagationmechanism can be compared to convective detonation (Ershov, FizikaGoreniya i Vzryva 1997; 33(1):98-106; Ershov, et al., Fizika Goreniya iVzryva 2001; 37(2):94-102) for flame propagation in a porous medium witha thin surface layer of explosive.

Another experimental technique aimed at the assessment of the combustionperformance of reactive nanocomposite materials is based on a pressuremeasurement in a confined chamber or pressure cell (Puszynski, et al.,Materials Research Society Symposium Proceedings 2006; 896:147-58;Prakash, et al., Nano Letters 2005; 5(7):1357-60; Perry, et al., Journalof Applied Physics 2007; 101(6):064313/1-064313/5; Puszynski, et al.,Journal of Propulsion and Power 2007; 23(4): 698-706; Schoenitz, et al.,Proceedings of the Combustion Institute 2005; 30:2071-8; Moore, et al.,Journal of Propulsion and Power 2007; 23(1):181-5). Standard vessels foroxygen bomb calorimetry (Perry, et al., Journal of Applied Physics 2007;101(6):064313/1-064313/5; Moore, et al., Journal of Propulsion and Power2007; 23(1): 181-5) as well as smaller (Puszynski, et al., MaterialsResearch Society Symposium Proceedings 2006; 896:147-58; Prakash, etal., Nano Letters 2005; 5(7):1357-60, Puszynski, et al., Journal ofPropulsion and Power 2007; 23(4): 698-706) and larger (Schoenitz, etal., Proceedings of the Combustion Institute 2005; 30:2071-8) vesselshave been used and combined with different igniters and pressuretransducers. In this approach, the experiment can be considered asnearly adiabatic and the pressure increase can be related to the energyproduced in combustion (Perry, et al., Journal of Applied Physics 2007;101(6):064313/1-064313/5). One substantial difficulty in interpretingthe experimental data is the production of transient, or semi-stable gasspecies. Examples of this kind of system are: Bi vapor in combustion ofthermites using Bi₂O₃ as an oxidizer (Puszynski, et al., MaterialsResearch Society Symposium Proceedings 2006; 896:147-58; Puszynski, etal., Journal of Propulsion and Power 2007; 23(4): 698-706), productionof hydrogen and water using hydrated oxides as oxidizers (Perry, et al.,Journal of Applied Physics 2007; 101(6):064313/1-064313/5), productionof nitrogen if nitrates are used as oxidizers (Umbrajkar, et al.,Propellants, Explosives, Pyrotechnics 2007; 32(1):32-41), etc.

Because combustion may not follow the equilibrium thermodynamicpredictions, thermodynamic calculations can only serve as an initialguideline for interpreting the experimental results. Despite suchdifficulties, it appears that comparison of different materials, ormaterials prepared from the same compounds but with different particlesizes or morphologies, can be very meaningful based on such pressuremeasurements. Both maximum pressures achieved and the rates of pressurerise are of interest. In addition, the final pressure in the chamberafter the reaction is completed and the products are cooled, is ofinterest as indicative of the final make-up of the gaseous reactionproducts. Finally, condensed reaction products can be readily recoveredand analyzed yielding information important for development of thereaction mechanism. Sets of related measurements with reactivenanocomposite materials prepared by ARM are presented by Umbrajkar, etal., Journal of Propulsion and Power 2008; 24(2)192-8 and Trunov, etal., Journal of Propulsion and Power 2008; 24(2):184-91. Because thesematerials comprise fully dense micron-sized particles that retain theirmorphology and structure upon being dispersed in a gaseous oxidizer, thecomposites ignited and burned uniformly when aerosolized within theexplosion vessel (Umbrajkar, et al., Journal of Propulsion and Power2008; 24(2)192-8; Trunov, et al., Journal of Propulsion and Power 2008;24(2):184-91). Thus, the pressure measurements were interpreted in termsof the flame propagation speed, in addition to the straightforwardinterpretation of the maximum achieved pressure as an indicator of theoverall energy release.

Details available in the literature are insufficient for meaningfulcomparison of different pressure measurements or other experimental datafor this type of experiments reported by different research groups. Itis suggested that future experiments in confined vessel experimentsspecify the cell volume, mass of the powder load, and the initial andfinal pressures obtained. Furthermore, the type of gas present in thepressure cell and the pressure increase obtained from the igniter itself(without the nanocomposite powder) would be useful for meaningfulcomparisons of the data from different investigators.

Measurements describing combustion of individual reactive nanocompositeparticles prepared by ARM were recently reported (Beloni, et al., AIAAPaper 2007-1431, 2007). The powder-like material was incorporated into aliquid fuel (decane) and the produced slurry was aerosolized using anultrasonic nozzle. The aerosol jet was burned in a lifted laminar flameconfiguration and the combustion of nanocomposite particles was studiedoptically. It was observed that micron-sized aluminum particles couldnot be ignited in this configuration; however, the same or coarser sizenanocomposite reactive particles with bulk composition 2B+Ti ignited andburned completely. The ignited particles were also observed todisintegrate during their combustion and continue burning as smallerfragments. This type of combustion is very attractive for practicalapplications where rapid burn rates are desired for particles that arecoarser and easier to work with than nanopowders.

Heterogeneous shock tube measurements were mentioned earlier as usefulfor finding ignition delays (Bazyn, et al., Combustion Science andTechnology 2007; 179:457-76). The same measurements are also useful forcharacterization of the ensuing combustion of the aerosolized material.If the material can survive the initial interaction with the incidentand reflected shock waves without being disintegrated, the informationobtained from detailed optical measurements can be used to determine theburn rates, combustion temperatures, and identify some of the productspecies formed, all of which is critically important for developingmeaningful combustion models. The issues of the material survival areeffectively removed when combustion of n-Al in gaseous oxidizers isstudied (Bazyn, et al., Combustion and Flame 2006; 145(4):703-13) usingthe heterogeneous shock tube technique. The results are very interestingand show substantial differences between combustion of n-Al andrelatively well characterized combustion of micron-sized Al particles.It was found that the burn time of n-Al particles decreases rapidly withthe increase in the ambient gas temperature. Furthermore, substantialreduction of the combustion time was also observed for n-Al at increasedpressures. These combustion features are indicative of a kinetic burningregime for n-Al.

Comparison of combustion features of micron- and nanosized Al powderswas also presented by Huang, et al., Proceedings of the CombustionInstitute 2007; 31(II):2001-9. Experiments employed a premixedBunsen-type flame with Al-laden flow fed from the burner's nozzle.Bimodal nano- and micron sized Al particles were used to produce laminarflames for which the speed and optical structure were studied andinterpreted theoretically.

Finally, detailed kinetics of gas phase combustion reactions issuccessfully addressed in a few well-instrumented laser ignitionexperiments employing extremely high heating rates (Yang, et al.,Journal of Physical Chemistry B 2003; 107(19):4485-93; Wang, et al.,Propellants, Explosives, Pyrotechnics 2005; 30(2): 148-55; Zamkov, etal., Journal of Physical Chemistry C 2007; 111(28): 10278-84; Mileham,et al., Journal of Physical Chemistry C 2007; 111(45):16883-8).

Performance in Practical Applications

Development of new reactive nanomaterials is driven by their potentialapplications in propellants, explosives, and pyrotechnics. Experimentalvalidations of the performance of related practical formulations are,therefore, very important for justifying further research and forguiding the material development efforts. Because reactive nanomaterialsbecame available only recently, the published results describing theirperformance are relatively scarce. Some of the first demonstrations ofthe effect of replacing micron-sized aluminum powder with its nano-sizedanalog for metallized solid propellants were described by Kuo, et al.,Materials Research Society Symposium Proceedings, 800 (Synthesis,Characterization and Properties of Energetic/Reactive Nanomaterials)Materials Research Society, p. 3-14, 2003). Recently, more experimentalstudies, e.g., (Dokhan, et al., Proceedings of the Combustion Institute2002; 29(2):2939-45; De Luca, et al., Fizika Goreniya i Vzryva 2005;41(6):80-94; Galfetti, et al., Journal of Physics: Condensed Matter2006; 18(33):S1991-S2005; Luman, et al., Proceedings of the CombustionInstitute 2007; 31(Pt. 2):2089-96; Zhang, et al., Huojian Tuijin 2006;32(1):35-9) and reviews (De Luca, Theory and Practice of EnergeticMaterials, Proceedings of the 7th International Autumn Seminar onPropellants, Explosives and Pyrotechnics, Xi'an, China, 2007, p.277-289; Beckstead, et al., Progress in Energy and Combustion Science2007; 33(6):497-551) were published. It is generally established thatthe burn rate and pressure exponent of the propellant formulations withn-Al can be improved compared to the standard metallized formulationsusing micron-sized powders. The improvement in the overall performance,however, depends on a broad range of the propellant characteristics,including oxygen balance of the formulation, particle size distributionsof other components, e.g., AP, the type of oxidizer used, for example APvs RDX and by other parameters, as discussed in detail in a recentreview (Beckstead, et al., Progress in Energy and Combustion Science2007; 33(6):497-551).

Applications of reactive nanomaterials in various pyrotechnic deviceshave also been investigated recently (Son, Materials Research SocietySymposium Proceedings 2003, 800:161-172; Higa, Journal of Propulsion andPower 2007; 23(4):722-7). Specifically, nanocomposite thermites wereconsidered as components for lead-free primers and substantial potentialfor this application has been recently demonstrated (Higa, Journal ofPropulsion and Power 2007; 23(4):722-7). Combustion of nanocompositethermite was found to produce gas pressures that are inadequate forignition primers; however, the combination of nanocomposite materialswith conventional or insensitive energetic formulations was found topresent a promising approach for future practical devices.

Applications of reactive nanomaterials as additives to liquid fuels(Beloni, et al., AIAA Paper 2007-1431, 2007), to enhanced blastexplosives (Lu, et al., 34th International Annual Conference of ICT2003; p. 128/1-128/14; Ward, et al., JANNAF Paper, San Diego Calif.2006; Gogulya, et al., Combustion, Explosion and Shock Waves 2008;44(2):198-212), and other compositions are also being actively explored.New application areas of reactive nanomaterials are also being explored.Such reactive materials are either expected to replace high explosivesin specific applications (Kim, et al., JANNAF Paper, Boston, Mass.2008), or replace inert structural components of the weapons systems(Ames, Materials Research Society Symposium Proceedings 2006;896:123-32).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is further illustrated in the following Examples, whichhowever are to be considered as exemplary and not definitive of theinvention.

Example 1

A SPEX 8000 shaker mill has been used extensively in current research onreactive milling and mechanical alloying [Suryanarayana, Progress inMaterials Science, 46 (2001) 1-184; Shoshin, et al., Comb. Flame 128(2002) 259-269; Schoenitz, et al., J. of Prop. and Power 19(3) (2003)405-412] and was also used in this and the other Examples. The SPEXshaker mill is a vibratory mill; its vial is agitated at high frequencyin a complex cycle that involves motion in three orthogonal directions.The reciprocating velocity of the vial in the SPEX 8000 series shakermill is directly proportional to the motor's rotational speed. Undervarious loading conditions, the rotational speed of the actuator inputshaft was measured with a stroboscope. The nominal rotational speed was1054 RPM, which translates to an oscillation frequency of 17.6 Hz. Thevial speed was not varied in the present Examples, but it offersadditional means of control over the milling process as the vial speeddirectly influences the impact velocity and frequency of collisions, andhence the energy transferred to the powder from the plastic deformation.Steel milling vials and balls were used in this and the remainingExamples. Milling media with higher or lower densities can also be usedto afford control over the collision energy between the media.

A thermistor was mounted on the milling vial and connected to a PC-baseddata logger to monitor its temperature. The spontaneous reactionregistered as a sharp temperature spike. Times of initiation weredetermined for varying milling parameters for each material. No processcontrol agent was used. The diameters of the balls used were 2.36, 3.16,4.76, and 9.52 mm. The materials were milled under argon. Theball-to-powder weight ratio (charge ratio, C_(R)) was set to 2.5, 5, and10.

Starting materials were Al (98%, 10-14 .mu.m), Fe₂O₃ (99.5%, −325 mesh),and MoO₃ (99.95%, −325 mesh) from Alfa Aesar. The total amount ofmaterial was 5 g in the case of Al—Fe₂O₃, and 2 g for Al—MoO₃. At agiven C_(R), this changes the number of balls used, and therefore themilling times required to initiate the spontaneous reaction fordifferent materials are not immediately comparable. The amount of theAl—MoO₃ mixture loaded in a single run did not exceed 2 g to avoiddamaging the milling vial because of the high local temperatures causedby the reaction.

After initiation times were determined, samples of metastable compositematerials were prepared using Arrested Reactive Milling (ARM) by haltingthe milling just before the initiation of the reaction. StoichiometricSamples of Al—Fe₂O₃ and Al—MoO₃ were prepared with varying millingtimes. The respective reproducibility of the initiation under identicalmilling conditions was found to be about 10% (see below). Therefore,samples arrested at approximately 90% of the time of spontaneousinitiation are designated as “fully milled” for reference. Partiallymilled samples were obtained at approximately 50% initiation time.

Materials Characterization

Powder x-ray diffraction was performed using a Philips X'pert MRD X-raydiffractometer. The surface morphology of individual particles as wellas the internal structure of cross-sectioned particles was investigatedusing a LEO 1530 Field Emission Scanning Electron Microscope (SEM).Cross sections were prepared by embedding small quantities of compositepowders in epoxy resin. The particles were embedded under vacuum toeliminate trapped gasses and avoid formation of bubbles. The mounts werethen polished by hand using successively finer SiC polishing paper up to1200 grit.

Particle size distributions for starting materials, mechanical alloysand their combustion products were determined by Low-Angle Laser LightScattering (LALLS) using a Coulter LS230 particle size analyzer.

Example 2 Combustion Testing

Several preliminary tests were carried out to assess changes in theignition and combustion behavior of the nano-composite thermitematerials produced by ARM. Schematic diagrams of the experimental setupsare shown in FIG. 1.

Ignition of the ARM-prepared powders was studied using an electricallyheated filament (FIG. 1 a). This technique has been describedextensively elsewhere (Trunov, et al., Chemical and Physical Processesin Combustion. The 2003 Technical Meeting of the Eastern States Sectionof the Combustion Institute University Park, Pa., (2003) pp. 313-316;Mohan, et al., Chemical and Physical Processes in Combustion. The 2003Technical Meeting of the Eastern States Section of the CombustionInstitute University Park, Pa., (2003) pp. 329-332). A thin layer ofpowder is coated on a conductive filament, which is electrically heatedat varying rates. Ignition of the powder is registered using aphotodiode focused on the powder coating. The temperature of thefilament is measured simultaneously with an infrared pyrometer focusedon an uncoated area of the filament adjacent to the powder. This setupminimizes errors due to unknown emissivities of different powders. Fromthe ignition temperatures measured in this setup at different heatingrates, the activation energy can be estimated by treatment analogous toisoconversion methods used in conventional thermal analysis (Starink,Thermochimica Acta 288 (1996) 97-104).

In another test, reaction rates were compared for nano-composite powdersprepared using ARM and blended initial component powders. In thesetests, the thermite powder was placed on a ceramic support inside aclosed pressure vessel equipped with a pressure transducer (FIG. 1 b).Prior to the experiment, the vessel was purged with argon. The powdercharge was ignited using a heated wire. The powder was not dispersedprior to ignition. However, most of the powder was airborne during thecombustion due to the expansion of surrounding and pore gas as well asdue to the production of intermediate volatile products. Pressure traceswere recorded, and combustion products were collected for x-ray phaseanalysis.

Linear burning rates of different powders were measured in the testillustrated in FIG. 1 c. The powders were placed in an open, rectangulargroove of 2.5.times.2.5 mm cross-section cut into a block of ceramic.Samples were initiated on one end with an electrically heated wire. Thepropagation of the combustion front was recorded with a high-speed videocamera at 500 frames per second.

Observed times of spontaneous initiation during milling of Al—MoO₃ andAl—Fe₂O₃ are shown in Table 1. The values and errors shown are theresults of 2-4 repetitions under identical conditions. Thereproducibility is found to be on the order of .+−. 10%. Milling timesgenerally decrease with increasing charge ratio C_(R).

TABLE 1 Milling times (minutes) required for spontaneous initiation ofstoichiometric mixtures of Al—MoO₃ and Al—Fe₂O₃ for specific ball sizesand charge ratios. Ball size, mm C_(R) = 2.5 C_(R) = 5 C_(R) = 10Al—MoO₃ 2.36 39.2 ± 1.1 33.2 ± 1.8 7.75 ± 0.21 3.16 24.2 ± 2.4 13.9 ±0.1 7.05 ± 0.07 4.76 22.8 ± 0.8 11.4 ± 0.6 5.85 ± 0.07 9.52 35.4 ± 6.49.65 ± 0.9 4.60 ± 0.14 Al—Fe₂O₃ 2.36 169 ± 94 14.8 ± 1.1 8.93 ± 0.643.16 59.0 ± 7.1 18.1 ± 0.5 9.47 ± 0.39 4.76 41.6 ± 4.1 20.5 ± 3.6 10.0 ±1.03 9.52 33.6 ± 4.3 11.7 ± 4.3 9.42 ±. 0.74

Recently, it has been suggested that the progress of mechanical alloyingor reactive milling can be described using the specific milling dose,D_(m), introduced as

D _(m) ˜I≈(n _(coll) ·E _(coll) ·t)/m _(p)  (1)

where I is the milling intensity, n_(coll) is the frequency of ball-ballcollisions, E_(coll) the averaged energy per collisions, t the millingtime, and m_(p) the powder mass (Delogu, et al., Chemical EngineeringScience 58 (2003) 815-821). It was assumed that the value of D_(m)determines the state of the milled material, and that ignition istriggered at a specific degree of grain refinement. Further simplifyingassumptions can be made, i.e., E_(coll)·˜m_(b), where m_(b) is the massof a single ball, and n_(coll)˜n_(b), where n_(b) is the number ofballs. With the time of the initiation, t_(init), this leads to aconstant milling dose, D_(m)* corresponding to a certain degree of grainrefinement:

D _(m) *˜n _(b) ·m _(b) ·t _(init) /m _(p) =C _(R) ·t _(init)  (2)

where the definition for the charge ratio C_(R)=n_(b) m_(b)/m_(p) wasused. Thus, for a certain degree of refinement, it is expected that

C_(R)t_(init)=const.  (3)

This reasoning suggests that the milling time required to triggerinitiation depends on the diameter of the milling balls exclusively vs.the charge ratio. A similar relation was suggested earlier (Delogu, etal. Chemical Engineering Science 58 (2003) 815-821). However, theanalysis of mechanical alloying processes presented in (Delogu, et al.,Chemical Engineering Science 58 (2003) 815-821) was based on a kineticexpression for very low degrees of milling-induced amorphization, andled to the conclusion that for a given milling state C_(R) ²t, should beconstant.

The product of the measured milling times leading to initiation and thecharge ratios, C_(R)t_(init), is plotted as a function of the balldiameter in FIG. 2. The values of C_(R)t_(init) for series withdifferent C_(R) superimpose and do not change significantly. Thus, tofirst approximation, the present observations appear to support thetentative trend as expected from Eq. (3), rather than constant values ofC_(R) ²t as suggested in Delogu, op. cit. Significant deviations fromconstant behavior predicted by Eq. (3), or even linearity exist,however, especially for smaller ball diameters where milling times aregreater than expected. The values of C_(R)t_(init) are slightlydifferent for Al—MoO₃ and Al—Fe₂O₃. While it is expected that differentmaterials require mixing of the components on different length scalesfor initiation to occur, the observed dependence on the ball diameter isnot intuitive. Both the achieved degree of structural and compositionalrefinement and the collision energy determined by the ball diametercould be important for initiation.

Example 3

Based on the good reproducibility of the experiments with balls of 4.76mm diameter, reactive composites were prepared for further analysis withthese balls and with C_(R)=5. “Fully milled” materials were milled for11 and 19 min for the Al—MoO₃ and Al—Fe₂O₃ mixtures, respectively.“Partially milled” materials were milled for about half the maximumtime, 6 and 10 min, respectively. In addition, fully reactive B—Tinano-composite powders were prepared by milling elemental B and Tipowders in steel vials for 150 min using steel balls with diameters of4.76 mm and a C_(R) of 5. Thermite powder blends were prepared bymanually homogenizing the starting materials under acetone for referencetests.

Materials Properties

SEM images of the fully milled nano-composite particles and particlecross-sections are shown in FIG. 3. The particles' surface morphology istypical for mechanically alloyed powders. The images of the particlecross-sections show that the initially spherical Al is thinned out tolayers of 10-100 nm thickness sandwiched between layers (MoO₃) ornanosized particles (Fe₂O₃) of oxidizer. XRD showed no structuralchanges in either Al or the respective oxides, although noticeable peakbroadening was observed for all materials indicating a reduction incrystallite sizes. Particle size distributions measured using LALLS areshown in FIG. 4. While the shape of the distribution changes, theaverage particle size of the reactive composite is close to the size ofthe starting materials. The size reduction implied by the LALLS data forpartially milled Al—MoO₃ is not supported by direct inspection of theparticles by microscopy, however. The partially milled particles areoblate; therefore, the light scattering-based measurement is likely toproduce erroneous results. The shapes of the fully milled/reactiveparticles are sufficiently close to spherical to justify the use ofLALLS.

Example 4 Preliminary Ignition and Combustion Testing

Ignition temperatures measured at different heating rates for the threetypes of fully reactive nanocomposite materials prepared using ARM areshown in Table 2. These data, processed using an isoconversion method(Starink, Thermochimica Acta 288 (1996) 97-104) to estimate the ignitionactivation energy, are plotted in FIG. 5. For comparison, similarlymeasured and processed experimental data describing ignition of pure Aland Mg powders in air (Trunov, et al., Chemical and Physical Processesin Combustion. The 2003 Technical Meeting of the Eastern States Sectionof the Combustion Institute University Park, Pa., (2003) pp. 313-316;Mohan, et al. Chemical and Physical Processes in Combustion. The 2003Technical Meeting of the Eastern States Section of the CombustionInstitute University Park, Pa., (2003) pp. 329-332) are also shown inFIG. 5. The activation energies for ignition of pure Al and Mg powdersin air are close to 215 kJ/mol and are noticeably higher than thosemeasured for the thermites and B—Ti nano-composites. The evaluatedactivation energies for the Al—Fe₂O₃ and Al—MoO₃ nano-composites are170.+−0.25 kJ/mol and 152.+−0.19 kJ/mol, respectively, and are close toeach other. These values are close to the activation energy of 167.5kJ/mol reported for the Al—Fe₂O₃ thermite reaction in (Maximov, et al.,Zhurnal Fizicheskoi Khimii 40(2) (1966) 467-470 (in Russian)). Theactivation energy for ignition of B—Ti nano-composites is significantlylower, 59.+−0.15 kJ/mol. In general, the activation energies areexpected to be unaffected by the nano-composite mixing of theingredients using ARM. However, the activation energies of the thermitecompositions prepared by mixing passivated nano-powders could besomewhat higher due to the passivating (e.g., oxide) layers.

TABLE 2 Ignition temperatures of fully milled Al—MoO₃, Al—Fe₂O₃ and B—Tinano-composites. Heating Rate, K/s Ignition Temperature, K Al—MoO₃ 3096 1104 972 1027 340 995 Al—Fe₂O₃ 3438  1249 883 1207 291 1110 B₅₀Ti₅₀ 412600 1335  678 5732  732

Example 5

Pressure traces measured in the constant volume explosion vessel forcombustion of different charges of thermite powders are shown in FIG. 6.While only qualitative conclusions can be drawn from the comparisons ofthe pressure traces measured for different powders, samples wereportioned to provide a constant 17.9 kJ of energy (4.50 g for Al—Fe₂O₃,and 3.81 g for Al—MoO₃ compositions). Powder blends, fully and partiallyreactive nano-composite powders prepared using ARM with differentmilling times were used in these tests. A summary of the measuredpressures and rates of pressure rise for different samples is given inTable 3. A clear trend of the accelerated reaction rate for ARM-preparedmaterials is visible from the comparison of the pressure traces shown inFIG. 6 and from the results shown in Table 3. It is also seen that thehighest reaction rates are observed for fully milled powders of both,Al—Fe₂O₃ and Al—MoO₃ thermites. Finally, a higher reactivity for theAl—MoO₃ thermites compared to the Al—Fe₂O₃ thermites is generallyobserved.

TABLE 3 Summary of results for the constant volume explosion tests withfully and partially milled nano-composites and with blends of therespective starting materials. Shown are maximum recorded pressure,highest rate of pressure rise, and average rate of pressure risecalculated from the time between ignition and the pressure maximum. Thepowder charges were selected to maintain a constant theoretical reactionenthalpy for all the samples. (dP/dt)_(max), (dP/dt)_(avg), P_(max), atmatm/s atm/s Al—MoO₃, 3.81 g 100% (10 min) 3.07 30 18.7  50% (6 min) 2.7821 12.2 blend 2.51 12 7.1 Al—Fe₂O₃, 4.50 g 100% (19 min) 2.48 9.3 5.6 50% (10 min) 2.25 4.9 2.6 blend 1.85 1.4 1.1

The combustion products collected from the pressure vessel were analyzedby SEM and XRD. A representative backscattered electron image is shownin FIG. 7. Product particles consist of aluminum oxide with caps of therespective reduced metal. This feature appears to be independent ofparticle size, it is even observed in 10 nm particles in the airbornefraction. XRD showed that the bulk of aluminum oxide found in the Moproducts was .delta.*-Al₂O₃, while it was .sigma.-Al₂O₃ in the Feproducts. Small amounts of .alpha-Al.O₃ were found in both cases aswell. Increasing amounts of unreacted Al and Fe₂O₃ were detected in theproducts of partially milled and unmilled Al—Fe₂O₃ composites.

The same pressure vessel has recently been used for constant volumeaerosol explosion tests carried out in air with different mechanicallyalloyed powders (Schoenitz, et al., J. of Prop. and Power 19(3) (2003)405-412). Nano-composites prepared using boron and titanium powdersusing ARM were made and compared with the respective powder blends.Higher rates of pressure rise were reported for the nano-compositepowders and a higher degree of conversion of the metallic powders tooxides was observed. Interestingly, some borides (e.g., TiB or TiB₂)were detected in the combustion products of B—Ti powder blends but noborides were found in the combustion products of the ARM-prepared B—Tinano-composite powders. More details on these experiments are availableelsewhere (Schoenitz, et al., op. cit.).

The results of the linear burn measurements for Al—Fe₂O₃ nano-compositesprepared using ARM with different milling times as well as for theblended Al and Fe₂O₃ powders are shown in FIG. 8. The powder blend wassuccessfully ignited, but combustion did not propagate. The propagationwas generally faster and the flame speed was more uniform for the powderfor which the milling was halted just prior to the expected spontaneousreaction. These results generally confirm that faster reaction kineticsis achieved for materials prepared by ARM.

The foregoing demonstrates that the reactive milling of powders withvery high reaction enthalpies can be reproducibly arrested to producenovel energetic material powders with particles in the 1-100 .mu.m sizerange. Individual particles of these powders are fully dense. Thecomposition of each particle is identical to the bulk powdercomposition. The components are intimately mixed in three-dimensionalnano-structures and ready to react upon initiation. An experimentalparametric study of the arrested reactive milling established that for arange of sizes of the grinding balls used, a concept of a milling doseproportional to the product of the milling time and charge ratio can beused to approximately predict the time necessary to prepare themetastable nano-composites. Ignition temperatures of the preparedmaterials were measured and their activation energies of ignition wereevaluated. The activation energy obtained from these experiments for theAl—Fe₂O₃ nano-composite is consistent with the known activation energyof the Al—Fe₂O₃ thermite reaction. Higher reaction rates were observedin combustion tests conducted in a constant volume pressure vessel inargon for the ARM-prepared nano-composites of both Al—Fe₂O₃ and Al—MoO₃as compared to the respective blends of initial powders and partiallymilled powders. Linear burning rates were observed to increase for theARM-prepared powders as the time when the reactive milling was arrestedapproached the expected time of the spontaneous reaction in the millingvial.

While the present invention has been described in terms of specificembodiments thereof, it will be understood in view of the presentdisclosure, that numerous variations upon the invention are now enabledto those skilled in the art, which variations yet reside within thescope of the present teaching. Accordingly, the invention is to bebroadly construed, and limited only by the scope and spirit of theclaims now appended hereto.

1. A metastable nano-composite material produced by a method consistingessentially of: (a) reactively milling a mixture of powdered componentsthat spontaneously react at a known duration of said milling; (b)stopping said milling at a time at which said components arecompositionally homogenized on a nanoscale to produce a nanocompositepowder but prior to said known duration, and thereby before saidspontaneous reaction occurs; and (c) recovering as a product the milledpowder as a nanostructured composite for subsequent use by controllablyinitiating destabilization thereof.
 2. A metastable nano-compositematerial in accordance with claim 1 comprising particles in the 1-50 μmrange.
 3. A metastable nano-composite material in accordance with claim1 that is highly reactive.
 4. A metastable nano-composite material inaccordance with claim 1 that is compositionally homogenized.
 5. Ametastable nano-composite material produced by a method consistingessentially of: (a) selecting starting components as two or morepowdered materials capable of a highly exothermic reaction; (b)reactively milling said starting components to achieve homogeneity; (c)stopping said milling at a time at which said components arecompositionally homogenized on the nanoscale to produce a nanocompositepowder, but prior to initiation of said exothermic reaction; and (d)recovering as a product the milled powder as a metastable nano-compositefor subsequent use by controllably initiating destabilization thereof.6. A metastable nano-composite material in accordance with claim 5comprising particles in the 1-50 μm range.
 7. A metastablenano-composite material in accordance with claim 5 that is highlyreactive.
 8. A metastable nano-composite material in accordance withclaim 1 that is compositionally homogenized.
 9. A metastablenano-composite material substantially in powder form.
 10. A metastablenano-composite material according to claim 9 that undergoes anexothermic reaction more quickly or at a lower temperature thanmaterials previously available.
 11. A metastable nano-composite materialaccording to claim 9 that undergoes an exothermic reaction in at leastabout 10% shorter time than the time required for materials previouslyavailable to undergo a similar or the same reaction at a given heatflow.
 12. A metastable nano-composite material according to claim 9 thatbegins to react in an exothermic reaction at a temperature that is atleast about 10% lower than the temperature at which materials previouslyavailable begin to react in the same or a similar reaction at a givenheat flow.
 13. A metastable nano-composite material according to claim 9that begins to react in an exothermic reaction at a temperature that isat least about 50 or more degrees Kelvin lower than the temperature atwhich materials previously available begin to react in the same or asimilar reaction at a given heat flow.
 14. A metastable nano-compositematerial according to claim 9 that begins to react in an exothermicreaction at a temperature that is at least about 100 or more degreesKelvin lower than the temperature at which materials previouslyavailable begin to react in the same or a similar reaction at a givenheat flow.
 15. A metastable nano-composite material according to claim 9wherein most or all particles are between 1-100 microns in diameter. 16.A metastable nano-composite material according to claim 9 wherein atleast about half of particles are smaller than about 50 microns indiameter
 17. A metastable nano-composite material according to claim 9wherein at least about half of particles are smaller than about 30microns in diameter.
 18. A metastable nano-composite material accordingto claim 9 wherein most or all inclusions in the material are about50-200 nanometers in diameter.
 19. A metastable nano-composite materialaccording to claim 9 that is 25% or more denser than similar materialsof the same elements produced by other means.
 20. A metastablenano-composite material according to claim 9 that substantiallycompletes an ignition reaction in at least 10% less time than the timerequired for similar materials of the same elements produced by othermeans at a given heating rate to substantially complete an ignitionreaction.
 21. A metastable nano-composite material according to claim 9that substantially completes an ignition reaction in at least 25% lesstime than the time required for similar materials of the same elementsproduced by other means at a given heating rate to substantiallycomplete an ignition reaction.